<PAGE>
 
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
 
                               ----------------
 
                                 SCHEDULE 14D-1
                                
                             (AMENDMENT NO. 4)     
 
                   TENDER OFFER STATEMENT PURSUANT TO SECTION
                14(D)(1) OF THE SECURITIES EXCHANGE ACT OF 1934
 
                              GRUMMAN CORPORATION
- --------------------------------------------------------------------------------
                           (NAME OF SUBJECT COMPANY)
 
                           NORTHROP ACQUISITION, INC.
- --------------------------------------------------------------------------------
                                    (BIDDER)
 
                    Common Stock, $1.00 par value per share
                       (Including the Associated Rights)
- --------------------------------------------------------------------------------
                         (TITLE OF CLASS OF SECURITIES)
 
                                    40018110
- --------------------------------------------------------------------------------
                     (CUSIP NUMBER OF CLASS OF SECURITIES)
 
                            Richard R. Molleur, Esq.
                              Northrop Corporation
                             1840 Century Park East
                             Los Angeles, CA 90067
                                 (310) 553-6262
 
                                    COPY TO:
 
                             Karen E. Bertero, Esq.
                            Gibson, Dunn & Crutcher
                             333 South Grand Avenue
                             Los Angeles, CA 90071
                                 (213) 229-7000
- --------------------------------------------------------------------------------
                 (NAME, ADDRESS AND TELEPHONE NUMBER OF PERSON
     AUTHORIZED TO RECEIVE NOTICES AND COMMUNICATIONS ON BEHALF OF BIDDER)
 
                           CALCULATION OF FILING FEE
- --------------------------------------------------------------------------------

<TABLE>
<CAPTION>
           TRANSACTION VALUATION*                       AMOUNT OF FILING FEE**
<S>                                                     <C>
              $2,104,030,800.00                              $420,806.16
</TABLE>

- --------------------------------------------------------------------------------
 
*  For purposes of calculating fee only. Assumes purchase of 35,067,180 shares
   of Common Stock, $1.00 par value per share, of Grumman Corporation at $60.00
   per share.
 
** 1/50th of 1% of Transaction valuation.
 
[_]CHECK BOX IF ANY PART OF THE FEE IS OFFSET AS PROVIDED BY RULE 0-11(A)(2)
   AND IDENTIFY THE FILING WITH WHICH THE OFFSETTING FEE WAS PREVIOUSLY PAID.
   IDENTIFY THE PREVIOUS FILING BY REGISTRATION STATEMENT NUMBER, OR THE FORM
   OR SCHEDULE AND THE DATE OF ITS FILING.
 
Amount previously paid: Not Applicable              Filing party: Not Applicable
                                                       
 
Form or registration no.: Not Applicable            Date filed: Not Applicable
                                                        
 
                               Page 1 of 4 Pages

<PAGE>
 
   
  This Amendment No. 4 amends and supplements the Tender Offer Statement on
Schedule 14D-1, dated March 14, 1994 (the "Schedule 14D-1"), of Northrop
Acquisition, Inc. (the "Purchaser") and Northrop Corporation ("Northrop"), as
amended, filed in connection with the Purchaser's offer to purchase all of the
outstanding shares of Common Stock, par value $1.00 per share, of Grumman
Corporation, a New York corporation (the "Company"), and the associated
preferred stock purchase rights, as set forth in the Schedule 14D-1.     
 
ITEM 3. PAST CONTACTS, TRANSACTIONS OR NEGOTIATIONS WITH THE SUBJECT COMPANY.
   
  On March 28, 1994, Northrop sent a letter to the Company, a copy of which is
attached hereto as Exhibit (c)(8) and incorporated herein by reference in its
entirety.     
 
ITEM 11. MATERIAL TO BE FILED AS EXHIBITS.
            
  (c)(8)   Letter dated March 28, 1994 from Northrop to the Company.     
 
                               Page 2 of 4 Pages

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                                   SIGNATURE
 
  After due inquiry and to the best of my knowledge and belief, I certify that
the information set forth in this statement is true, complete and correct.
   
Dated: March 29, 1994     
 
                                        NORTHROP CORPORATION
 
                                        /s/ Richard R. Molleur
                                        ----------------------------------------
                                        Name: Richard R. Molleur
                                        Title: Corporate Vice President
 
                                        NORTHROP ACQUISITION, INC.
 
                                        /s/ Richard R. Molleur
                                        ----------------------------------------
                                        Name: Richard R. Molleur
                                        Title: Vice President and Secretary
 
                               Page 3 of 4 Pages

<PAGE>
 
                                 EXHIBIT INDEX
 
 EXHIBIT
   NO.                               DESCRIPTION
 -------                             -----------
   
  (c)(8)  Letter dated March 28, 1994 from Northrop to the Company.     
         

                               Page 4 of 4 Pages





<PAGE>
 
                                       March 28, 1994



Dr. Renso L. Caporali
Chairman and Chief Executive Officer
Grumman Corporation
1111 Stewart Avenue
Bethpage, NY 11714-3580

Dear Renso:

We have received, and are reviewing, your letter dated March 28, 1994.

I wish to point out that the process you have proposed continues the unlevel
playing field.

First, Northrop continues to be faced with the cost of the Martin Marietta
lockup fee, a $50 plus million expense that has been imposed on Northrop but not
(obviously) on Martin Marietta.

Second, the high bid presently "on the table" is Northrop's bid of $60 per
share. Martin Marietta has been aware of that fact for over two weeks. By
contrast, Northrop is now being asked to bid against a purely hypothetical price
increase from Martin Marietta without any notice or knowledge of whether such an
increase is in fact submitted. In essence, Grumman is proposing that Northrop
bid against itself. Since Northrop has fiduciary duties to its own
stockholders, this imposes a considerable burden upon us.

While the Grumman Board has "determined that it is in the best interests of the 
shareholders of the Company to have free and open competitive bidding," the 
proposed process is not open. While we share Grumman's desire to
 avoid undue 
delay in concluding the present acquisition process, it is not in the best 
interests of Grumman's stockholders to prematurely cut off potential bidding and
potentially deprive stockholders of the opportunity to receive the maximum value
for their shares. This, of course, will occur if Northrop is not allowed the 
opportunity to top any increase in bid which Martin Marietta wishes to make.
 


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Page 2

 
A process, similar to that which you have proposed, might have been appropriate
in January before actions were taken to exclude Northrop from the bidding which
forced us to act publicly. Having done so, it is apparent that, without the
bidding rules and procedures described in your March 28 letter, the current
process is open (all correspondence, offers, etc., must be public); it is fair
(there can be no backroom deals); it is orderly (the SEC regulations guide the
process); it will conclude in the very near future (both suitors are subject to
time and financial constraints); and, it is in the best interest of all
concerned--the shareholders, employees and communities benefit precisely
because the inherent transparency of the process means that the highest offer
will win. This then is what we believe is a true, open process in which we are
currently engaged.

                                       Sincerely,

                                       /s/ KENT KRESA
                                       Kent Kresa