UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549


                                    FORM 10-K

(Mark One)
|X| ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE SECURITIES EXCHANGE ACT
OF 1934 For the fiscal year ended: December 31, 2006
                                       or
|_| TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE SECURITIES
   EXCHANGE ACT OF 1934
For Transition period ______________________________


             Commission File Number of issuing entity: 333-131201-01
                           RFMSI SERIES 2006-S4 TRUST
           (Exact name of issuing entity as specified in its charter)

                 RESIDENTIAL FUNDING MORTGAGE SECURITIES I, INC.
              (Exact name of depositor as specified in its charter)

                        RESIDENTIAL FUNDING COMPANY, LLC
               (Exact name of sponsor as specified in its charter)

                                    New York
                       ------------------------ ---------
      (State or other jurisdiction of incorporation or organization of the
                                 issuing entity)


            c/o Residential Funding Company, LLC, as Master Servicer
                         8400 Normandale Lake Boulevard
                          Minneapolis, Minnesota 55437

           (Address of principal executive offices of issuing entity)

             (952) 857-7000 (Telephone number, including area code)

                                      None
                      (I.R.S. Employer Identification No.)

                                       N/A
           (Former name, former address, if changed since last report)

Securities registered pursuant to Section 12(b) of the Act: None
Securities Registered pursuant to Section 12(g) of the Act: None


Indicate by check mark if the  registrant is a well-known  seasoned  issuer,  as
defined in Rule 405 of the Securities Act.
Yes |_|  No  |X|
Indicate  by  check  mark if the  registrant  is not  required  to file  reports
pursuant to Section 13 or Section 15(d) of the Act.

<PAGE>

Yes |X|  No  |_|
Indicate by check mark whether the registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during
the  preceding 12 months (or for such  shorter  period that the  registrant  was
required  to file  such  reports),  and  (2) has  been  subject  to such  filing
requirements for the past 90 days.
|X| Yes           |_| No
Indicate by check mark if disclosure of delinquent  filers  pursuant to Item 405
of Regulation  S-K is not contained  herein,  and will not be contained,  to the
best of registrant's  knowledge,  in definitive proxy or information  statements
incorporated  by reference in Part II of the Form 10-K or any  amendment to this
Form 10-K. |X| [Item 405 of Regulation S-K is not applicable.] Indicate by check
mark whether the registrant is a large accelerated  filer, an accelerated filer,
or a  non-accelerated  filer.  See  definition of  "accelerated  filer and large
accelerated  filer"  in Rule  12b-2 of the  Exchange  Act.  (Check  one):  Large
accelerated filer |_| Accelerated filer |_|  Non-accelerated  filer |X| Indicate
by check mark  whether  the  registrant  is a shell  company (as defined in Rule
12b-2 of the Act). Yes |_| No |X| Registrant has no voting or non-voting  common
equity outstanding held by non-affiliates.


<PAGE>


                                     PART I

THE FOLLOWING ITEMS HAVE BEEN OMITTED IN ACCORDANCE  WITH GENERAL  INSTRUCTION J
TO FORM 10-K:

Item 1: Business.
Item 1A: Risk Factors.
Item 2: Properties.
Item 3: Legal Proceedings.
Item 4: Submission of Matters to a Vote of Security Holders.

ITEM 1B. UNRESOLVED STAFF COMMENTS.

Not Applicable.

SUBSTITUTE INFORMATION PROVIDED IN ACCORDANCE WITH GENERAL INSTRUCTION J TO FORM
10-K:

ITEM 1112(B) OF REGULATION AB. SIGNIFICANT OBLIGORS OF POOL ASSETS
(FINANCIAL INFORMATION).

None.

ITEM 1114(B)(2) OF REGULATION AB: CREDIT ENHANCEMENT AND OTHER SUPPORT,
EXCEPT FOR CERTAIN DERIVATIVES INSTRUMENTS (FINANCIAL INFORMATION).

None.

ITEM 1115(B) OF REGULATION AB: CERTAIN DERIVATIVES INSTRUMENTS (FINANCIAL
INFORMATION).

Based  on the  standards  set  forth  in  Item  1115(b)  of  Regulation  AB,  no
information is required in response to this Item.

ITEM 1117 OF REGULATION AB: LEGAL PROCEEDINGS.

There are no material pending legal or other proceedings  involving the mortgage
loans or Residential Funding Company,  LLC ("Residential  Funding"),  as sponsor
and master  servicer,  Residential  Funding  Mortgage  Securities  I,  Inc.,  as
depositor,  RFMSI  Series  2006-S4  Trust,  as the issuing  entity,  Homecomings
Financial,   LLC   ("Homecomings")   and  GMAC  Mortgage,   LLC  ("GMACM"),   as
subservicers,  or other  parties  described in Item 1117 of  Regulation AB that,
individually  or in the  aggregate,  would  have a  material  adverse  impact on
investors in the securities (the "Securities") to which this report relates.


<PAGE>

Residential  Funding,  Homecomings  and GMACM are  currently  parties to various
legal  proceedings  arising  from time to time in the  ordinary  course of their
businesses,  some of which  purport to be class  actions.  Based on  information
currently available,  it is the opinion of Residential Funding,  Homecomings and
GMACM that the  eventual  outcome of any  currently  pending  legal  proceeding,
individually  or in the  aggregate,  will not have a material  adverse effect on
their ability to perform their obligations in relation to the mortgage loans. No
assurance,  however,  can be  given  that  the  final  outcome  of  these  legal
proceedings, if unfavorable,  either individually or in the aggregate, would not
have a material  adverse impact on Residential  Funding,  Homecomings and GMACM.
Any such  unfavorable  outcome could adversely affect the ability of Residential
Funding,  Homecomings and GMACM to perform its servicing  duties with respect to
the  mortgage  loans and  potentially  lead to the  replacement  of  Residential
Funding, Homecomings and GMACM with a successor servicer.



                                     PART II

THE FOLLOWING ITEMS HAVE BEEN OMITTED IN ACCORDANCE  WITH GENERAL  INSTRUCTION J
TO FORM 10-K:

Item 5:  Market for Registrant's Common Equity, Related Stockholder Matters
         and Issuer Purchases of Equity Securities.
Item 6:  Selected Financial Data.
Item 7:  Management's Discussion and Analysis of Financial Condition and
         Results of Operations.
Item 7A: Quantitative and Qualitative Disclosures about Market Risk.
Item 8:  Financial Statements and Supplementary Data.
Item 9:  Changes in and Disagreements with Accountants on Accounting and
         Financial Disclosure.
Item 9A: Controls and Procedures.

ITEM 9B: OTHER INFORMATION.

None.

                                    PART III

THE FOLLOWING ITEMS HAVE BEEN OMITTED IN ACCORDANCE  WITH GENERAL  INSTRUCTION J
TO FORM 10-K:

Item 10: Directors and Executive Officers of the Registrant.
Item 11: Executive Compensation.
Item 12: Security Ownership of Certain Beneficial Owners and Management and
         Related Stockholder Matters.
Item 13: Certain Relationships and Related Transactions.
Item 14: Principal Accountant Fees and Services.


<PAGE>

SUBSTITUTE INFORMATION PROVIDED IN ACCORDANCE WITH GENERAL INSTRUCTION J TO FORM
10-K:

ITEM 1119 OF REGULATION AB:  AFFILIATIONS AND CERTAIN RELATIONSHIPS AND
RELATED TRANSACTIONS.

Information  contemplated  by Item 1119 of  Regulation  AB has been omitted from
this report in reliance on the Instruction to Item 1119.

ITEM 1122 OF REGULATION AB: COMPLIANCE WITH APPLICABLE SERVICING CRITERIA.

Each of Residential Funding Company,  LLC,  Homecomings  Financial LLC, Wachovia
Mortgage  Corporation,  ZC Sterling  Insurance Agency,  Inc., U.S. Bank National
Association  and Wells Fargo Bank,  National  Association  (each,  a  "Servicing
Participant") has been identified by the registrant as a party  participating in
the  servicing  function  with  respect to the pool  assets  held by the Issuing
Entity.  Each Servicing  Participant  has completed a report on an assessment of
compliance with the servicing criteria applicable to such Servicing  Participant
(each,  a "Report on  Assessment")  as of  December  31, 2006 and for the period
covered by this Form 10-K,  which Reports on Assessment are attached as exhibits
to this Form 10-K.  In  addition,  each  Servicing  Participant  has provided an
attestation report (each, an "Attestation  Report") by a registered  independent
public  accounting  firm  regarding  its  related  Report  on  Assessment.  Each
Attestation Report is attached as an exhibit to this Form 10-K.

A Servicing  Participant may engage one or more vendors to perform  specific and
limited,  or scripted  activities  that  address all or a portion of one or more
servicing criteria applicable to such Servicing Participant. In general in these
cases,  the Servicing  Participant  has  instituted  policies and  procedures to
monitor whether such vendors'  activities  comply in all material  respects with
such  servicing  criteria,  and may elect to take  responsibility  for assessing
compliance with the servicing criteria applicable to such vendors' activities in
such  Servicing   Participant's  Report  on  Assessment.   Where  the  Servicing
Participant  has not  instituted  such  policies  and  procedures,  or where the
Servicing  Participant  does  not  otherwise  elect to take  responsibility  for
assessing its vendors'  activities,  the vendor is itself treated as a Servicing
Participant  and is required to provide its own Report on Assessment and related
Attestation Report.

Except as disclosed below, no Report on Assessment or related Attestation Report
has identified  (i) any material  instance of  noncompliance  with the servicing
criteria  identified  in such Report on  Assessment as applicable to the related
Servicing  Participant  or  (ii)  any  material  deficiency  in  such  Servicing
Participant's policies and procedures to monitor vendor compliance.

Exceptions:

Residential  Funding  Company,   LLC:  The  Report  on  Assessment  prepared  by
Residential  Funding  Company,  LLC  and the  related  Attestation  Report  have
identified material noncompliance with one servicing criterion applicable to it.
Specifically,   with  regard  to  servicing  criterion  1122(d)(3)(i)(B)  (which
contemplates  that  reports  to  investors  provide  information  calculated  in
accordance with the terms specified in the transaction agreements),  information
required  by the  transaction  agreements  was  excluded  from,  or  incorrectly
reported in, certain of the investor  reports.  The registrant  does not believe
these  reporting  errors had any material  impacts or effects on  investors  and
corrective action was completed to address ongoing compliance.


<PAGE>

Platform-Level Reports:

Regulations of the Securities and Exchange  Commission  (the "SEC") require that
each  Servicing  Participant  complete a Report on  Assessment  at a  "platform"
level,  meaning that the transactions covered by the Report on Assessment should
include  all  asset-backed  securities  transactions  involving  such  Servicing
Participant  that are backed by the same asset type.  More recent  guidance from
the SEC staff identifies additional parameters which a Servicing Participant may
apply to define  and  further  limit its  platform.  For  example,  a  Servicing
Participant  may define its  platform  to include  only  transactions  that were
completed  on or after  January  1, 2006 and that were  registered  with the SEC
pursuant  to  the  Securities  Act  of  1933.  Each  Servicing   Participant  is
responsible  for defining its own platform,  and each  platform  will  naturally
differ based on various factors,  including the Servicing Participant's business
model and the transactions in which it is involved.

Wachovia  Mortgage  Corporation's  ("WMC")  Report  on  Assessment  defines  its
platform to include the asset-backed securities transactions for which it serves
as a servicer  involving first lien mortgage  loans.  Exhibit A to its Report on
Assessment  identifies  the  individual  asset-backed  transactions  defined  by
management as constituting  its platform,  but does not include the asset-backed
transaction to which this Form 10-K relates.

Neither  WMC's  Report on  Assessment  nor its  related  Attestation  Report has
identified  (i) any  material  instance  of  noncompliance  with  the  servicing
criteria  identified  in such Report on  Assessment as applicable to WMC or (ii)
any material  deficiency  in WMC's  policies and  procedures  to monitor  vendor
compliance.  WMC has  advised us that the  mortgage  loans  held by the  Issuing
Entity that WMC services are serviced in  accordance  with the same policies and
procedures,  and to the same  standards,  as it  applies to the  mortgage  loans
included in its platform.

Except as discussed above, based on our understanding of their platforms and the
guidance that is available at this time, we believe that the parameters by which
the Servicing  Participants  have defined their platforms should be permissible.
However,  because  the  SEC's  regulations  are  new and  the  guidance  that is
available at this time is subject to clarification  or change,  we cannot assure
you that the SEC and its staff will necessarily agree.

ITEM 1123 OF REGULATION AB: SERVICER COMPLIANCE STATEMENT.

Each of Residential  Funding  Company,  LLC, GMAC Mortgage,  LLC and Homecomings
Financial,  LLC (each, a "Servicer")  has been identified by the registrant as a
servicer  with  respect to the pool  assets  held by the  Issuing  Entity.  Each
Servicer has provided a statement of compliance with its  obligations  under the
servicing agreement  applicable to such Servicer (a "Compliance  Statement") for
the  period  covered  by this Form 10-K,  in each case  signed by an  authorized
officer of the related  servicer.  Each  Compliance  Statement is attached as an
exhibit to this Form 10-K. None of the Compliance  Statements has identified any
instance where the related Servicer has failed to fulfill its obligations  under
the applicable servicing agreement in any material respects.


                                     PART IV

ITEM 15.    EXHIBITS, FINANCIAL STATEMENT SCHEDULES.

      (a)(1) Not Applicable.

      (a)(2) Not Applicable.

      (a)(3) Not Applicable.

      (b) Exhibits.


<PAGE>

EXHIBIT
NUMBER                                  DESCRIPTION
----------   ------------------------------------------------------------------
3.1          Certificate of Incorporation of Residential Funding Mortgage
             Securities I, Inc. (incorporated by reference to the Annual
             Report on Form 10-K for the period ended December 31, 2006,
             filed by RFMSI Series 2006-S1 Trust with the Securities and
             Exchange Commission).
3.2          Bylaws of Residential Funding Mortgage Securities I, Inc.
             (incorporated by reference to the Annual Report on Form 10-K for
             the period ended December 31, 2006, filed by RFMSI Series
             2006-S1 Trust with the Securities and Exchange Commission).
10.1         Series  Supplement,  dated as of April 1,  2006,  and the  Standard
             Terms of Pooling and  Servicing  Agreement,  dated as of January 1,
             2006, among  Residential  Funding  Mortgage  Securities I, Inc., as
             company,  Residential  Funding Company,  LLC (formerly  Residential
             Funding  Corporation),  as master servicer,  and U.S. Bank National
             Association, as trustee (incorporated by reference to the Report on
             Form 8-K  filed by the  Issuing  Entity  with  the  Securities  and
             Exchange Commission on May 12, 2006).
10.2         Assignment and Assumption Agreement,  dated as of April 27, between
             Residential  Funding  Company,  LLC (formerly  Residential  Funding
             Corporation) and Residential  Funding  Mortgage  Securities I, Inc.
             (incorporated  by  reference to the Report on Form 8-K filed by the
             Issuing Entity with the  Securities and Exchange  Commission on May
             12, 2006).
31.1         Certification of Residential Funding Company,  LLC pursuant to Rule
             13a-14(d)/15d-14(d) under the Securities Exchange Act of 1934.
33.1         Report on  Assessment  of  Compliance  with  Servicing  Criteria of
             Residential  Funding  Company,  LLC as of December 31, 2006 and for
             the period covered by this Form 10-K.
33.2         Report on  Assessment  of  Compliance  with  Servicing  Criteria of
             Homecomings  Financial,  LLC as of  December  31,  2006 and for the
             period covered by this Form 10-K.
33.3.1       Report on  Assessment  of  Compliance  with  Servicing  Criteria of
             Wachovia  Mortgage  Corporation as of December 31, 2006 and for the
             period covered by this Form 10-K.
33.3.2       Report on Assessment of Compliance with Servicing Criteria of ZC
             Sterling Insurance Agency, Inc. as of December 31, 2006 and for
             the period covered by this Form 10-K.
33.4         Report on Assessment of Compliance with Servicing  Criteria of U.S.
             Bank  National  Association  as of  December  31,  2006 and for the
             period covered by this Form 10-K.
33.5         Report on Assessment of Compliance with Servicing Criteria of Wells
             Fargo Bank,  National  Association  as of December 31, 2006 and for
             the period covered by this Form 10-K.
34.1         Attestation Report of  PricewaterhouseCoopers  LLP on Assessment of
             Compliance with Servicing Criteria relating to Residential  Funding
             Company, LLC.
34.2         Attestation Report of  PricewaterhouseCoopers  LLP on Assessment of
             Compliance   with  Servicing   Criteria   relating  to  Homecomings
             Financial, LLC.
34.3.1       Attestation  Report of KPMG LLP on Assessment  of  Compliance  with
             Servicing Criteria relating to Wachovia Mortgage Corporation.
34.3.2       Attestation Report of Ernst &Young, LLP on Assessment of Compliance
             with Servicing  Criteria relating to ZC Sterling  Insurance Agency,
             Inc.
34.4         Attestation Report of Ernst & Young LLP on Assessment of
             Compliance with Servicing Criteria relating to U.S. Bank
             National Association.
34.5         Attestation Report of KPMG LLP on Assessment of Compliance with
             Servicing Criteria relating to Wells Fargo Bank, National
             Association.
35.1         Servicer Compliance Statement of Residential Funding Company,
             LLC.
35.2         Servicer Compliance Statement of GMAC Mortgage, LLC.
35.3         Servicer Compliance Statement of Homecomings Financial, LLC.

      (c) Not Applicable.



<PAGE>



                                   SIGNATURES

Pursuant to the  requirements of Section 13 or 15(d) of the Securities  Exchange
Act of 1934,  the  registrant  has duly  caused  this report to be signed on its
behalf by the undersigned, thereunto duly authorized.

Date:  March 26, 2007
                                        RFMSI SERIES 2006-S4 TRUST

                                        By:  Residential Funding Company,
                                            LLC,
                                              as Master Servicer

                                              By:  /s/ Anthony N.  Renzi
                                                  Name: Anthony N. Renzi
                                                  Title:  Managing Director






<PAGE>


                                INDEX TO EXHIBITS

EXHIBIT
NUMBER                                  DESCRIPTION
----------   ------------------------------------------------------------------
3.1          Certificate of Incorporation of Residential Funding Mortgage
             Securities I, Inc. (incorporated by reference to the Annual
             Report on Form 10-K for the period ended December 31, 2006,
             filed by RFMSI Series 2006-S1 Trust with the Securities and
             Exchange Commission).
3.2          Bylaws of Residential Funding Mortgage Securities I, Inc.
             (incorporated by reference to the Annual Report on Form 10-K for
             the period ended December 31, 2006, filed by RFMSI Series
             2006-S1 Trust with the Securities and Exchange Commission).
10.1         Series  Supplement,  dated as of April 1,  2006,  and the  Standard
             Terms of Pooling and  Servicing  Agreement,  dated as of January 1,
             2006, among  Residential  Funding  Mortgage  Securities I, Inc., as
             company,  Residential  Funding Company,  LLC (formerly  Residential
             Funding  Corporation),  as master servicer,  and U.S. Bank National
             Association, as trustee (incorporated by reference to the Report on
             Form 8-K  filed by the  Issuing  Entity  with  the  Securities  and
             Exchange Commission on May 12, 2006).
10.2         Assignment and Assumption Agreement,  dated as of April 27, between
             Residential  Funding  Company,  LLC (formerly  Residential  Funding
             Corporation) and Residential  Funding  Mortgage  Securities I, Inc.
             (incorporated  by  reference to the Report on Form 8-K filed by the
             Issuing Entity with the  Securities and Exchange  Commission on May
             12, 2006).
31.1         Certification of Residential Funding Company,  LLC pursuant to Rule
             13a-14(d)/15d-14(d) under the Securities Exchange Act of 1934.
33.1         Report on  Assessment  of  Compliance  with  Servicing  Criteria of
             Residential  Funding  Company,  LLC as of December 31, 2006 and for
             the period covered by this Form 10-K.
33.2         Report on  Assessment  of  Compliance  with  Servicing  Criteria of
             Homecomings  Financial,  LLC as of  December  31,  2006 and for the
             period covered by this Form 10-K.
33.3.1       Report on  Assessment  of  Compliance  with  Servicing  Criteria of
             Wachovia  Mortgage  Corporation as of December 31, 2006 and for the
             period covered by this Form 10-K.
33.3.2       Report on Assessment of Compliance with Servicing Criteria of ZC
             Sterling Insurance Agency, Inc. as of December 31, 2006 and for
             the period covered by this Form 10-K.
33.4         Report on Assessment of Compliance with Servicing  Criteria of U.S.
             Bank  National  Association  as of  December  31,  2006 and for the
             period covered by this Form 10-K.
33.5         Report on Assessment of Compliance with Servicing Criteria of Wells
             Fargo Bank,  National  Association  as of December 31, 2006 and for
             the period covered by this Form 10-K.
34.1         Attestation Report of  PricewaterhouseCoopers  LLP on Assessment of
             Compliance with Servicing Criteria relating to Residential  Funding
             Company, LLC.
34.2         Attestation Report of  PricewaterhouseCoopers  LLP on Assessment of
             Compliance   with  Servicing   Criteria   relating  to  Homecomings
             Financial, LLC.
34.3.1       Attestation  Report of KPMG LLP on Assessment  of  Compliance  with
             Servicing Criteria relating to Wachovia Mortgage Corporation.
34.3.2       Attestation Report of Ernst &Young, LLP on Assessment of Compliance
             with Servicing  Criteria relating to ZC Sterling  Insurance Agency,
             Inc.
34.4         Attestation Report of Ernst & Young LLP on Assessment of
             Compliance with Servicing Criteria relating to U.S. Bank
             National Association.
34.5         Attestation Report of KPMG LLP on Assessment of Compliance with
             Servicing Criteria relating to Wells Fargo Bank, National
             Association.
35.1         Servicer Compliance Statement of Residential Funding Company,
             LLC.
35.2         Servicer Compliance Statement of GMAC Mortgage, LLC.
35.3         Servicer Compliance Statement of Homecomings Financial, LLC.