EXHIBIT 99.4
 
                           FORM OF LETTER TO CLIENTS
          AND FORM OF INSTRUCTION TO BOOK-ENTRY TRANSFER PARTICIPANTS
 
                                   OFFER FOR
           ALL OUTSTANDING 9 5/8% SENIOR SUBORDINATED NOTES DUE 2008
                                IN EXCHANGE FOR
               9 5/8% SERIES B SENIOR SUBORDINATED NOTES DUE 2008
                        WHICH HAVE BEEN REGISTERED UNDER
                     THE SECURITIES ACT OF 1933, AS AMENDED
                                       OF
                         INSIGHT HEALTH SERVICES CORP.
 
To Our Clients:
 
    We are enclosing herewith a Prospectus, dated       , 1998 of InSight Health
Services Corp. (the "Issuer") and a related Letter of Transmittal (which
together constitute the "Exchange Offer") relating to the offer by the Issuer to
exchange its 9 5/8% Series B Senior Subordinated Notes Due 2008 (the "Exchange
Notes"), pursuant to an offering registered under the Securities Act of 1933, as
amended (the "Securities Act"), for a like principal amount of its issued and
outstanding 9 5/8% Senior Subordinated Notes Due 2008 (the "Outstanding Notes")
upon the terms and subject to the conditions set forth in the Exchange Offer.
 
    PLEASE NOTE THAT THE EXCHANGE OFFER WILL EXPIRE AT 5:00 P.M., NEW YORK CITY
TIME, ON       , 1998, UNLESS EXTENDED.
 
    The Exchange Offer is not conditioned upon any minimum number of Outstanding
Notes being tendered.
 
    We are the participant in a book-entry transfer facility of Outstanding
Notes held by us for your account. A tender of such Outstanding Notes can be
made only by us as the participant in such book-entry transfer facility and
pursuant to your instructions. The Letter of Transmittal is furnished to you for
your information only and cannot be used by you to tender Outstanding Notes held
by us for your account.
 
    We request instructions as to whether you wish to tender any or all of the
Outstanding Notes held by us for your account pursuant to the terms and
conditions of the Exchange Offer. We also request that you confirm that we may
on your behalf make the representations contained in the Letter of Transmittal
that are to be made with respect to you as beneficial owner.
 
    Pursuant to the Letter of Transmittal, each holder of Outstanding Notes will
represent to the Issuer that (i) the Exchange Notes acquired in the Exchange
Offer are being obtained in the ordinary course of business of the person
receiving such Exchange Notes, (ii) the holder of the Outstanding Notes has no
arrangement or understanding with any person to participate in the distribution
of such Exchange Notes, (iii) if the holder is not a broker-dealer or is a
broker-dealer but will not receive Exchange Notes for its own account in
exchange for Outstanding Notes, the holder is not engaged in and does not intend
to participate in a distribution of the Exchange Notes and that it has no
arrangement or understanding with respect to the distribution of the Exchange
Notes and (iv) the holder is not an "affiliate" of the Issuer within the meaning
of Rule 405 under the Securities Act. If the tendering holder is a broker-dealer
that will receive Exchange Notes for its own account pursuant to the Exchange
Offer, we will represent on behalf of such broker-dealer that the Outstanding
Notes to be exchanged for the Exchange Notes were acquired by it as a result of
market-making activities or other trading activities, and acknowledge on behalf
of such broker-dealer that it will deliver a prospectus meeting the requirements
of the Securities Act in connection with any resale of such Exchange Notes. By
acknowledging that it will deliver and by delivering a prospectus meeting the
requirements of the Securities Act in connection with any resale of such
Exchange Notes, such broker-dealer will not be deemed to admit that it is an
"underwriter" within the meaning of the Securities Act.
 
Very truly yours,

                                 INSTRUCTION TO
                 BOOK-ENTRY TRANSFER PARTICIPANT FROM OWNER OF
                         INSIGHT HEALTH SERVICES CORP.
                   9 5/8% SENIOR SUBORDINATED NOTES DUE 2008
 
To Participant of the Book Entry Transfer Facility:
 
    The undersigned hereby acknowledges receipt of the Prospectus dated       ,
1998 of InSight Health Services Corp. (the "Issuer") and a related Letter of
Transmittal (which together constitute the "Exchange Offer"). Capitalized terms
used but not defined herein have the meanings ascribed to them in the
Prospectus.
 
    This will instruct you, the book-entry transfer facility participant, as to
the action to be taken by you relating to the Exchange Offer with respect to the
Outstanding Notes held by you for the account of the undersigned.
 
    The aggregate face amount of the Outstanding Notes held by you for the
account of the undersigned is (fill in amount):
 
$         of the 9 5/8% Senior Subordinated Notes Due 2008.
 
With respect to the Exchange Offer, the undersigned hereby instructs you (check
appropriate statement):
 
A.       To TENDER the following Outstanding Notes held by you for the account
of the undersigned (insert principal amount of Outstanding Notes to be
tendered): $         of the 9 5/8% Senior Subordinated Notes Due 2008, and not
to tender other Outstanding Notes, if any, held by you for the account or the
undersigned;
 
OR
 
B.       NOT to tender any Outstanding Notes held by you for the account of the
undersigned.
 
    If the undersigned instructs you to tender the Outstanding Notes held by you
for the account of the undersigned, it is understood that you are authorized to
make, on behalf of the undersigned (and the undersigned, by its signature below,
hereby makes to you), the representations and warranties contained in the Letter
of Transmittal that are to be made with respect to the undersigned as a
beneficial owner, including but not limited to; the representations that (i) the
Exchange Notes acquired pursuant to the Exchange Offer are being obtained in the
ordinary course of business of the undersigned, (ii) the undersigned has no
arrangement nor understanding with any person to participate in the distribution
of such Exchange Notes, (iii) if the undersigned is not a broker-dealer or is a
broker-dealer but will not receive Exchange Notes for its own account in
exchange for Outstanding Notes, the undersigned is not engaged in and does not
intend to participate in a distribution of the Exchange Notes and that it has no
arrangement or understanding with respect to the distribution of the Exchange
Notes and (iv) the undersigned is not an "affiliate" of the Issuer within the
meaning of Rule 405 under the Securities Act. If the undersigned is a
broker-dealer (whether or not it is also an "affiliate") that will receive
Exchange Notes for its own account pursuant to the Exchange Offer, it represents
that such Outstanding Notes to be exchanged for the Exchange Notes were acquired
by it as a result of market-making activities or other trading activities, and
it acknowledges that it will deliver a prospectus meeting the requirements of
the Securities Act in connection with any resale of such Exchange Notes. By
acknowledging that it will deliver and by delivering a prospectus meeting the
requirements of the Securities Act in connection with any resale of such
Exchange Notes, the undersigned will not be deemed to admit that it is an
"underwriter" within the meaning of the Securities Act.

                                   SIGN HERE
 
Signature(s): __________________________________________________________________
 
Name(s) (please print): ________________________________________________________
 
Address: _______________________________________________________________________
 
Telephone Number: ______________________________________________________________
 
Taxpayer Identification or Social Security Number: _____________________________
 
Date: __________________________________________________________________________