<PAGE>   1

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                                 UNITED STATES

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                   FORM 10-Q
(MARK ONE)

[X]   QUARTERLY REPORT UNDER SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT
      OF 1934

FOR THE QUARTERLY PERIOD ENDED                 SEPTEMBER 30, 1999

                                       OR

[ ]   TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
     SECURITIES EXCHANGE ACT OF 1934

FOR THE TRANSITION PERIOD FROM  _____________________ TO
 _____________________  .

COMMISSION FILE NO. 33-13437

                         DEL TACO INCOME PROPERTIES IV
                        A CALIFORNIA LIMITED PARTNERSHIP
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

<TABLE>
<S>                                                          <C>
                         CALIFORNIA                                       33-0241855
              (STATE OR OTHER JURISDICTION OF                          (I.R.S. EMPLOYER
               INCORPORATION OR ORGANIZATION)                       IDENTIFICATION NUMBER)

   23041 AVENIDA DE LA CARLOTA, LAGUNA HILLS, CALIFORNIA                     92653
          (ADDRESS OF PRINCIPAL EXECUTIVE OFFICES)                        (ZIP CODE)
</TABLE>

                                 (949) 462-9300
              (REGISTRANT'S TELEPHONE NUMBER, INCLUDING AREA CODE)

     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.  YES  X   NO  ___

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                                      INDEX

                          DEL TACO INCOME PROPERTIES IV

<TABLE>
<CAPTION>
PART I.  FINANCIAL INFORMATION                                       PAGE NUMBER
<S>                                                                  <C>
Item 1.  Financial Statements and Supplementary Data

Balance Sheets at September 30, 1999 (Unaudited) and
     December 31, 1998                                                     3

Statements of Income for the three and nine months ended
     September 30, 1999 and 1998 (Unaudited)                               4

Statements of Cash Flows for the nine months ended
     September 30, 1999 and 1998 (Unaudited)                               5

Notes to Financial Statements                                              6

Item 2.  Management's Discussion and Analysis of
         Financial Condition and Results of Operations                     8


PART II. OTHER INFORMATION

Item 6.  Exhibits and Reports on Form 8-K                                 10


SIGNATURES                                                                11
</TABLE>


                                      -2-
<PAGE>   3

                          DEL TACO INCOME PROPERTIES IV

                                 BALANCE SHEETS

<TABLE>
<CAPTION>
                                                     SEPTEMBER 30,   December 31,
                                                        1999            1998
                                                     -------------   ------------
                                                     (UNAUDITED)
<S>                                                   <C>            <C>
                                       ASSETS
CURRENT ASSETS:
     Cash                                             $   99,034     $  101,404
     Receivable from General Partner (Note 4)             26,575        106,724
     Deposits                                                426            504
                                                      ----------     ----------
        Total current assets                             126,035        208,632
                                                      ----------     ----------

PROPERTY AND EQUIPMENT, AT COST:
     Land and improvements                             1,236,700      1,236,700
     Buildings and improvements                        1,289,860      1,289,860
     Machinery and equipment                             484,789        484,789
                                                      ----------     ----------
                                                       3,011,349      3,011,349
     Less--accumulated depreciation                      999,105        921,294
                                                      ----------     ----------
                                                       2,012,244      2,090,055
                                                      ----------     ----------
                                                      $2,138,279     $2,298,687
                                                      ==========     ==========

                          LIABILITIES AND PARTNERS' EQUITY

CURRENT LIABILITIES:
     Payable to Limited Partners                      $   18,954     $   18,684
     Accounts payable                                      4,767          5,271
                                                      ----------     ----------
        Total current liabilities                         23,721         23,955
                                                      ----------     ----------

OBLIGATION TO GENERAL PARTNER                            137,953        137,953
                                                      ----------     ----------

PARTNERS' EQUITY:
     Limited Partners                                  1,987,973      2,146,545
     General Partner-Del Taco, Inc.                      (11,368)        (9,766)
                                                      ----------     ----------
                                                       1,976,605      2,136,779
                                                      ----------     ----------
                                                      $2,138,279     $2,298,687
                                                      ==========     ==========
</TABLE>

                          The accompanying notes are an
                   integral part of these financial statements


                                      -3-
<PAGE>   4

                          DEL TACO INCOME PROPERTIES IV

                              STATEMENTS OF INCOME

                                   (UNAUDITED)

<TABLE>
<CAPTION>
                                       THREE MONTHS ENDED       NINE MONTHS ENDED
                                          SEPTEMBER 30,           SEPTEMBER 30,
                                        1999       1998         1999         1998
                                      -------     -------     --------     --------
<S>                                   <C>         <C>         <C>          <C>
REVENUES:
     Rent (Notes 3 and 4)             $82,484     $83,729     $235,844     $236,540
     Interest                             518         447        2,512        1,102
     Other                                225         200          500          825
                                      -------     -------     --------     --------
                                       83,227      84,376      238,856      238,467
                                      -------     -------     --------     --------

EXPENSES:
     General and administrative         7,075       6,923       35,221       34,757
     Depreciation                      25,937      25,937       77,811       77,811
                                      -------     -------     --------     --------
                                       33,012      32,860      113,032      112,568
                                      -------     -------     --------     --------

        Net income                    $50,215     $51,516     $125,824     $125,899
                                      =======     =======     ========     ========

     Net income per limited
        partnership unit (Note 2)     $  0.30     $  0.31     $   0.75     $   0.75
                                      =======     =======     ========     ========
</TABLE>

                   The accompanying notes are an integral part
                         of these financial statements.


                                      -4-
<PAGE>   5

                          DEL TACO INCOME PROPERTIES IV

                            STATEMENTS OF CASH FLOWS

                                   (UNAUDITED)

<TABLE>
<CAPTION>
                                                             NINE MONTHS ENDED
                                                               SEPTEMBER 30,
                                                            1999           1998
                                                         ---------      ---------
<S>                                                      <C>            <C>
CASH FLOWS FROM OPERATING ACTIVITIES:

Net income                                               $ 125,824      $ 125,899
Adjustments to reconcile net income to net
  cash provided by operating activities:
     Depreciation                                           77,811         77,811
     Decrease in receivable from General Partner            80,149         38,123
     Decrease (increase) in deposits                            78           (208)
     (Decrease) increase in accounts payable and
        payable to limited partners                           (234)        13,055
                                                         ---------      ---------

           Net cash provided by operating activities       283,628        254,680
                                                         ---------      ---------

CASH FLOWS FROM FINANCING ACTIVITIES:

Cash distributions to partners                            (285,998)      (225,848)
                                                         ---------      ---------

Net (decrease) increase in cash                             (2,370)        28,832

Beginning cash balance                                     101,404         75,411
                                                         ---------      ---------

Ending cash balance                                      $  99,034      $ 104,243
                                                         =========      =========
</TABLE>

                   The accompanying notes are an integral part
                         of these financial statements.


                                      -5-
<PAGE>   6

                          DEL TACO INCOME PROPERTIES IV

                          NOTES TO FINANCIAL STATEMENTS

                               SEPTEMBER 30, 1999

NOTE 1 - BASIS OF PRESENTATION

The accompanying financial statements, some of which are unaudited, have been
prepared in accordance with generally accepted accounting principles for interim
financial information and with the instructions to Form 10-Q and Article 10 of
Regulation S-X. Accordingly, they do not include all of the information and
footnotes required by generally accepted accounting principles for complete
financial statements and should therefore be read in conjunction with the
financial statements and notes thereto contained in the partnership's annual
report on Form 10-K for the year ended December 31, 1998. In the opinion of
management, all adjustments (consisting of normal recurring accruals) necessary
to present fairly the partnership's financial position at September 30, 1999,
the results of operations and cash flows for the nine month periods ended
September 30, 1999 and 1998 have been included. Operating results for the three
and nine months ended September 30, 1999 are not necessarily indicative of the
results that may be expected for the year ending December 31, 1999.

NOTE 2 - NET INCOME PER LIMITED PARTNERSHIP UNIT

Net income per limited partnership unit is based upon the weighted average
number of units outstanding during the periods presented which amounted to
165,375 and 165,415 in 1999 and 1998, respectively.

Pursuant to the partnership agreement, annual partnership income or loss is
allocated one percent to the General Partner and 99 percent to the limited
partners. Partnership gains from any sale or refinancing will be allocated one
percent to the General Partner and 99 percent to the limited partners until
allocated gains and profits equal losses, distributions and syndication costs,
and until each class of limited partners receive their priority return as
defined in the partnership agreement. Additional gains will be allocated 12
percent to the General Partner and 88 percent to the limited partners.


                                      -6-
<PAGE>   7

                          DEL TACO INCOME PROPERTIES IV

                    NOTES TO FINANCIAL STATEMENTS - CONTINUED

                               SEPTEMBER 30, 1999

NOTE 3 - LEASING ACTIVITIES

The partnership leases certain properties for operation of restaurants to Del
Taco, Inc. on a triple net basis. The leases are for terms of 32 years
commencing with the completion of the restaurant facility located on each
property and require monthly rentals equal to 12 percent of the gross sales of
the restaurants. There is no minimum rental under any of the Leases.

For the three months ended September 30, 1999, the two restaurants operated by
Del Taco, for which the partnership is the lessor, had combined, unaudited sales
of $442,290 and net income of $42,179 as compared to $443,637 and $33,419
respectively, for the corresponding period in 1998. Net income by restaurant
includes charges for general and administrative expenses incurred in connection
with supervision of restaurant operations and interest expense. For the three
months ended September 30, 1999, the one restaurant operated by a Del Taco
franchisee, for which the partnership is the lessor, had unaudited sales of
$245,067 as compared with $254,096 during the same period in 1998.

For the nine months ended September 30, 1999, the two restaurants operated by
Del Taco, for which the partnership is the lessor, had combined, unaudited sales
of $1,266,114 and net income of $95,660 as compared to $1,251,044 and $80,268
respectively, for the corresponding period in 1998. For the nine months ended
September 30, 1999, the one restaurant operated by a Del Taco franchisee, for
which the partnership is the lessor, had unaudited sales of $699,250 as compared
with $720,128 during the same period in 1998.

NOTE 4 - TRANSACTIONS WITH DEL TACO

The receivable from the General Partner consists primarily of rent accrued for
the month of September. The September rent was collected on October 13, 1999.

Del Taco, Inc. serves in the capacity of general partner in other partnerships
which are engaged in the business of operating restaurants, and three other
partnerships which were formed for the purpose of acquiring real property in
California for construction of Mexican-American restaurants for lease under
long-term agreements to Del Taco, Inc. for operation under the Del Taco trade
name.

In addition, see Note 5 with respect to certain distributions to the General
Partner.

NOTE 5 - DISTRIBUTIONS

On October 12, 1999, a distribution to the limited partners of $74,329 or
approximately $.45 per limited partnership unit, was approved. Such distribution
was paid October 18, 1999. The General Partner also received a distribution of
$751 with respect to its 1% partnership interest.


                                      -7-
<PAGE>   8

ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
        OF OPERATIONS

Liquidity and Capital Resources

The partnership offered limited partnership units for sale between June 1987 and
June 1988. 14.5% of the $4.135 million raised through sale of limited
partnership units was used to pay commissions to brokers and to reimburse the
General Partner for offering costs incurred. Approximately $3 million of the
remaining funds were used to acquire sites and build three restaurants. In
February of 1992, approximately $442,000 raised during the offering but not
required to acquire sites and build restaurants was distributed to the limited
partners.

The three restaurants leased to Del Taco make up almost all of the income
producing assets of the partnership. Therefore, the business of the partnership
is almost entirely dependent on the success of the Del Taco trade name
restaurants that lease the properties. The success of the restaurants is
dependent on a large variety of factors, including, but not limited to, consumer
demand and preference for fast food, in general, and for Mexican-American food
in particular.

Results of Operations

The partnership owns three properties that are under long-term lease to Del Taco
for restaurant operations (Del Taco, in turn, has sub-leased one of the
restaurants to a Del Taco franchisee).

The following table sets forth rental revenue earned by restaurant for the year:

<TABLE>
<CAPTION>
                                       THREE MONTHS ENDED         NINE MONTHS ENDED
                                           SEPTEMBER 30,            SEPTEMBER 30,
                                         1999        1998         1999         1998
                                       -------     -------     --------     --------
<S>                                    <C>         <C>         <C>          <C>
Orangethorpe Ave., Placentia, CA       $35,240     $36,025     $102,786     $102,047

Lakeshore Drive, Lake Elsinore, CA      29,409      30,492       83,910       86,415

Highland Ave., San Bernardino, CA       17,835      17,212       49,148       48,078
                                       -------     -------     --------     --------

        Total                          $82,484     $83,729     $235,844     $236,540
                                       =======     =======     ========     ========
</TABLE>

The partnership receives rental revenues equal to 12 percent of gross sales from
the restaurants. The partnership earned rental revenue of $82,484 during the
three month period ended September 30, 1999, which represents a decrease of
$1,245 from 1998. The partnership earned rental revenue of $235,844 during the
nine month period ended September 30, 1999, which represents a decrease of $696
from 1998. The changes in rental revenue between 1999 and 1998 are directly
attributable to changes in sales levels at the restaurants under lease.


                                      -8-
<PAGE>   9

ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
        OF OPERATIONS - CONTINUED

The following table breaks down general and administrative expenses by type of
expense:

<TABLE>
<CAPTION>
                                            Percentage of Total
                                       General & Administrative Expense
                                             Nine Months Ended
                                               September 30,
                                       --------------------------------
                                            1999           1998
                                          --------       -------
<S>                                         <C>           <C>
Accounting fees                             61.63%        64.40%
Distribution of
  information to
  limited partners                          38.37         35.60
                                           ------        ------
                                           100.00%       100.00%
</TABLE>

General and administrative costs for the nine month period ended September 30,
increased from 1998 to 1999 due to increased costs for accounting and income tax
return preparation.

For the three months ended September 30, 1999, net income decreased by $1,301
from 1998 to 1999 due to the decrease in revenues of $1,149 and by the $152
increase in general and administrative expenses. For the nine months ended
September 30, 1999, net income decreased by $75 from 1998 to 1999 due to the
increase in revenues of $389 which was offset by the $464 increase in general
and administrative expenses.

The General Partner does not believe the operations of the partnership will be
significantly impacted by the year 2000 software issue and does not believe the
year 2000 software issue will materially effect the partnerships operations,
financial position or cash flows.


                                      -9-
<PAGE>   10

PART II. OTHER INFORMATION

Item 6. Exhibits and Reports on Form 8-K

(b) No reports on Form 8-K were filed during the nine months ended September 30,
1999.


                                      -10-
<PAGE>   11

                                   SIGNATURES

Pursuant to the requirements 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.

                                            DEL TACO INCOME PROPERTIES IV
                                            (a California limited partnership)
                                            Registrant

                                            Del Taco, Inc.
                                            General Partner

Date: October 29, 1999                      /s/ Robert J. Terrano
                                            -----------------------------------
                                            Robert J. Terrano
                                            Executive Vice President,
                                            Chief Financial Officer

Date: October 29, 1999                      /s/ C. Douglas Mitchell
                                            -----------------------------------
                                            C. Douglas Mitchell
                                            Vice President and Corporate
                                            Controller

                                      -11-
<PAGE>   12

                                 EXHIBIT INDEX


<TABLE>
<S>   <C>
27    Financial Data Schedule
</TABLE>