<PAGE>

                                                                     EXHIBIT 4.2

                          FORM OF SUBSCRIPTION WARRANT

                                      WESTCORP

                      SUBSCRIPTION WARRANT FOR RIGHTS OFFERING
                      FOR HOLDERS OF RECORD ON FEBRUARY 9, 2002

<Table>
<S>                                             <C>                  <C>
  ------------------------------------------                           ------------------------------------------
         SUBSCRIPTION WARRANT NUMBER                                                  CUSIP NUMBER
  ------------------------------------------    ------------------     ------------------------------------------
         SHARES ELIGIBLE TO SUBSCRIBE                 RIGHTS                       RECORD DATE SHARES
</Table>

     Westcorp (the "Company") is conducting a rights offering (the "Rights
Offering") which entitles the holders of shares of the Company's common stock
(the "Common Stock"), as of the close of business on February 9, 2002 (the
"Record Date") to receive one transferable right (each, a "Right") for each
share of Common Stock held of record on the Record Date. Holders of Rights are
entitled to subscribe for and purchase one share of Common Stock for every 10.5
Rights (the "Basic Subscription Right") at a subscription price of $14.67 per
share. If any shares of Common Stock are not purchased by holders of Rights
pursuant to the Basic Subscription Right (the "Excess Shares"), any holder
purchasing all of the shares of Common Stock available to that holder may
purchase an additional number of the Excess Shares, if so specified in the
subscription documents, subject to proration. No fractional shares or cash in
lieu thereof will be issued or paid. Set forth above is the number of shares of
Common Stock held by such holder, and the number of whole shares to which each
holder is entitled to subscribe pursuant to the Basic Subscription Right
(rounded down, if applicable, to the nearest whole share).

     For a more complete description of the terms and conditions of the Rights
Offering, please refer to the Prospectus dated January 22, 2002 (the
"Prospectus"), which is incorporated herein by reference. Copies of the
Prospectus are available upon request from Mellon Investor Services LLC (toll
free (866) 825-8874).

     This Subscription Warrant (or a Notice of Guaranteed Delivery) must be
received by Mellon Investor Services LLC together with payment in full of the
subscription price by 5:00 p.m. New York City time, on March 5, 2002 (unless
extended in the sole discretion of the Company) (as it may be extended, the
"Expiration Date"). Any Rights not exercised prior to the Expiration Date will
be null and void. Any subscription for shares of Common Stock in the Rights
Offering made hereby is irrevocable.

     The Rights represented by this Subscription Warrant may be exercised by
duly completing Form 1; may be transferred, assigned, exercised or sold through
a bank or broker by duly completing Form 2; and may be sold through Mellon
Investor Services LLC by duly completing Form 3. Each of these forms is set
forth on the reverse hereof. Rights holders are advised to review the Prospectus
and instructions, copies of which are available from Mellon Investor Services
LLC, before exercising or selling their Rights.

SUBSCRIPTION PRICE: $14.67 PER SHARE
<PAGE>

     The registered owner whose name is inscribed hereon, or its assigns, is
entitled to subscribe for shares of Common Stock of the Company upon the terms
and subject to the conditions set forth in the Prospectus and the instructions
relating to the use hereof.

     The Subscription Warrant is transferable, and may be combined or divided at
the office of Mellon Investor Services LLC. If the number of transferred Rights
would otherwise allow the purchase of a fractional share, the number of shares
which may be purchased must be rounded down to the nearest whole share (or any
lesser number of whole shares) that may be purchased with that number of Rights.

     Rights holders should be aware that if they choose to exercise or transfer
only part of their Rights, they may not receive a new Subscription Warrant in
sufficient time to exercise the remaining Rights evidenced thereby.

                                        2
<PAGE>

                                     FORM 1
                      (ON REVERSE OF SUBSCRIPTION WARRANT)

EXERCISE AND SUBSCRIPTION: The undersigned hereby irrevocably exercises one or
more Rights to subscribe for shares of Common Stock as indicated below, on the
terms and subject to the conditions specified in the Prospectus, receipt of
which is hereby acknowledged.

     (a) Number of whole shares subscribed for pursuant to the Basic
         Subscription Right  ____________ X $14.67 = $  ________ payment. (10.5
         Rights needed to subscribe for one share.)

     (b) Number of whole shares subscribed for pursuant to the Oversubscription
         Right  ____________ X $14.67 = $  ________ payment.

     (c) Total Subscription (sum of payment amounts on lines (a) and (b)) =
         $  ________ payment.*

METHOD OF PAYMENT (CHECK AND COMPLETE APPROPRIATE BOX(ES)):

[ ] Check, bank draft, or U.S. postal money order payable to "Mellon Investor
    Services LLC, as Subscription Agent" or

[ ] Wire transfer directed to The Chase Manhattan Bank, New York, NY, ABA No.
    021000021, Mellon Investor Services, LLC, MISC Reorg. Control A/C
    323-885489 -- (Westcorp).

     (d) If the Rights being exercised pursuant to the Basic Subscription Right
         do not constitute all of the Rights represented by the Subscription
         Warrants (check only one):

[ ] Deliver to the undersigned a new Subscription Warrant evidencing the
    remaining Rights to which the undersigned is entitled.

[ ] Deliver a new Subscription Warrant in accordance with the undersigned's Form
    2 instructions (which include any required signature guarantees).

[ ] Sell the remaining unexercised Rights in accordance with the undersigned's
    Form 3 instructions.

[ ] Do not deliver any new Subscription Warrants to me.

     (e) [ ] Check here if Rights are being exercised pursuant to the Notice of
Guaranteed Delivery delivered to the Subscription Agent prior to the date hereof
and complete the following:

    Name(s) of Registered Holder(s)
    Window Ticket Number (if any)
    Date of Execution of Notice of Guaranteed Delivery
    Name of Institution Which Guaranteed Delivery
---------------
* If the aggregate Subscription Price enclosed or transmitted is insufficient to
  purchase the total number of shares included in lines (a) and (b), or if the
  number of shares being subscribed for is not specified, the Rights holder
  exercising this Subscription Warrant shall be deemed to have subscribed for
  the maximum amount of shares that could be subscribed for upon payment of such
  amount. If the number of shares to be subscribed for pursuant to the
  Oversubscription Right is not specified and the amount enclosed or transmitted
  exceeds in aggregate the Subscription Price for all shares represented by this
  Subscription Warrant (the "Subscription Excess"), the Rights holder exercising
  this Subscription Warrant shall be deemed to have exercised the
  Oversubscription Right to purchase, to the extent available, that number of
  whole shares of Common Stock equal to the

                                        3
<PAGE>

  quotient obtained by dividing the Subscription Excess by the Subscription
  Price, subject to proration as described in the Prospectus. To the extent any
  portion of the aggregate Subscription Price enclosed or transmitted remains
  after the foregoing procedures, such funds shall be mailed to the subscriber
  with interest if the aggregate Subscription Price is tendered in immediately
  available funds, as described in the Prospectus, and without deduction as soon
  as practicable.

Subscriber's Signature  __________________ Telephone No. ( ____ )  _____________

                                        4
<PAGE>

                                     FORM 2
                      (ON REVERSE OF SUBSCRIPTION WARRANT)

TO TRANSFER YOUR SUBSCRIPTION WARRANT OR SOME OR ALL OF YOUR RIGHTS, OR TO
EXERCISE OR SELL RIGHTS THROUGH YOUR BANK OR BROKER: For value received, Rights
represented by this Subscription Warrant are hereby assigned to (please print in
full name and address and Taxpayer Identification Number or Social Security
Number of transferee):

Name:

Address:

Signature(s) of Transferee(s)

Signatures Guaranteed by:

     Proceeds from the sale of Rights may be subject to withholding of U.S.
taxes unless the Seller's certified U.S. taxpayer identification number (or
certificate regarding foreign status) is on file with the Subscription Agent and
the seller is not otherwise subject to U.S. backup withholding.

                                     FORM 3
                      (ON REVERSE OF SUBSCRIPTION WARRANT)

TO SELL SOME OR ALL OF YOUR UNEXERCISED RIGHTS THROUGH THE SUBSCRIPTION AGENT:

     The undersigned hereby authorizes the Subscription Agent to sell
 ____________ Rights represented by this Subscription Warrant but not exercised
hereby and to deliver to the undersigned a check for the proceeds, if any, from
the sale thereof, less any applicable brokerage commissions, taxes or other
direct expenses of sale. The Subscription Agent's obligation to execute orders
is subject to its ability to find buyers for the Rights.

Subscriber's Signature

     In order to sell Rights through the Subscription Agent, you must complete
and sign the substitute Form W-9 as provided in Section 8 of the instructions.

                                     FORM 4
                      (ON REVERSE OF SUBSCRIPTION WARRANT)

DELIVERY INSTRUCTIONS: Address for mailing of stock or new Subscription Warrant
or any cash payment in accordance with the Prospectus, if different from the
address shown on the face of this Subscription Warrant:

Name:

Address:

                                        5