A man who hid behind four identities in the last four years yesterday pleaded guilty to concealing evidence from police of the kidnaping of a Mexican businessman whose body was allegedly dismembered and dumped in the Chesapeake Bay four years ago.

Thurston Drew Shrader, who was arrested in Newport News, Va., posing as T. Alexander Marbury, a history teacher and popular basketball coach at a Newport News high school, entered his plea in U.S. District Court in Alexandria. The formal charge was misprison of a felony.

Charges that Shrader kidnaped Raul Hernandez-Bustamante at Dulles International Airport on Jan. 20, 1973, were dropped by the U.S. prosecutors.

Shrader "did not have advance knowledge of the impending kidnaping but only became involved after competion of the crime," according to a statement issued by U.S. Attorney William B. Cummings.

In an interview with The Post in May, Shrader said that his sister's boyfriend had been "ripped off" by Hernandez while he was living in Mexico.

The sister's boyfriend, Kenneth B. Krohn, a Harvard PhD., sought Shrader's help in luring Hernandez from Mexico to Dulles, Shrader alleged.

Shrader said he'd help Krohn because while Krohn was in Mexico Shrader had been using his identity to teach computer science at Severna Prep School in Severna Park, Shrader told The Post.

Shrader said that while using the name of Robert Graham, he called Hernandez several times, asking the businessman to meet him in Washington. Shrader said he accompanied Krohn to Dulles on Jan. 20, 1973 to meet Hernandez. Shrader said Krohn told him once Hernandez was in the country he would be arrested by federal authorities.

When Krohn left the airport he told Shrader that Hernandez had been arrested while changing planes in Dalles, Shrader said.

The following Tuesday Shrader said that he went to Krohn's Severna Park apartment and alleged that Krohn showed him Hernandez's head in the bathtub. The teeth, he said, were in the sink.

Plastic bags and a green foot locker in Krohn's apartment contained the rest of Hernandez, Shrader said Krohn told him.

Shrader said he then helped Krohn dump what he believed to be the body over the Chesapeake Bay Bridge after Krohn threatened his life and that of his girl friend.

Hernandez's body was never found.

Krohn has denied Shrader's allegations and said Shrader made up the story.

Both Krohn and Shrader have been arrested twice in connection with Hernandez's disappearance. The second time Krohn was arrested, in 1974, an Alexandria magistrate dismissed the charges for lack of evidence Krohn is now living in Cambridge Mass.