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Prospectus Supplement No. 4                     Filed Pursuant to Rule 424(b)(3)
to Prospectus Dated December 22, 1999           File No.        333-88985


                                 CEPHALON, INC.

       2,500,000 SHARES OF $3.625 CONVERTIBLE EXCHANGEABLE PREFERRED STOCK
             $125,000,000 7.25% CONVERTIBLE SUBORDINATED DEBENTURES
                        6,975,447 SHARES OF COMMON STOCK

         This prospectus supplement supplements information contained in that
certain prospectus of Cephalon, Inc. dated December 22, 1999, as amended and
supplemented from time to time, relating to the potential resale from time to
time of $3.625 Convertible Exchangeable Preferred Stock, par value $.01 per
share, 7.25% Convertible Subordinated Debentures (if issued) and Common Stock
issuable upon conversion of the Preferred Stock or Debentures (if issued). This
prospectus supplement is not complete without, and may not be delivered or
utilized except in connection with the prospectus, including any amendments or
supplements thereto.

         The following table supplements, or amends, as noted, the information
set forth in the prospectus under the caption "Selling Stockholders" with
respect to:

                  -        the respective amount of shares of preferred stock
                           beneficially owned by each selling stockholder and
                           that may be sold and the percent of outstanding
                           preferred stock held by such stockholder;

                  -        the principal amount of debentures that would be
                           beneficially owned and may be sold by each selling
                           stockholder; and

                  -        the number of shares of common stock that may be sold
                           by the selling stockholders pursuant to this
                           prospectus, as amended or supplemented.




                                                                                                                    Number of
                                                                                                                  Common Shares
                                               Number of Shares      Percent of        Principal Amount of         Issued Upon
                                              of Preferred Stock     Outstanding     Debentures Beneficially     Conversion That
           Name of                               Beneficially         Preferred         Owned That May Be          May Be Sold
   Selling Stockholder (1)                        Owned (2)             Stock                  Sold
   -----------------------                    ------------------     -----------     -----------------------     ---------------
                                                                                                     
The following information amends the
information set forth in the prospectus
originally filed:

Deutsche Bank Securities, Inc. ..........      193,800                7.8%            $9,690,000                  540,702

- -------------

(1)      The information set forth herein is as of January 11, 2000 and will be
         updated as required.

(2)      Beneficial ownership is determined in accordance with the rules of the
         SEC and generally includes voting or investment power with respect to
         securities.




                 THE DATE OF THIS PROSPECTUS IS JANUARY 12, 2000