Karen K. Johnson has told federal prosecutors that her former attorney, John A. Shorter Jr., helped arrange payments to her in exchange for her refusal to testify three years ago before a federal grand jury investigating alleged drug use by D.C. Mayor Marion Barry and others, according to sources.

Shorter, a 59-year-old lawyer who represented Johnson after she was indicted in 1984 on charges of possession of and conspiracy to sell cocaine, is the fourth person whom Johnson is believed to have identified in connection with the alleged payments.

The Washington Post reported yesterday that sources said Johnson had told prosecutors she received money from D.C. businessman John B. Clyburn and that she believes businessman Roy Littlejohn and perhaps others contributed funds. The sources said Johnson has also told prosecutors that her brother, Jared Kinnon, received some of the payments on her behalf while she was in jail and that Kinnon had given authorities a similar story.

Larry Gondelman, one of two lawyers representing Shorter on an appeal of his 1985 conviction on tax evasion charges, said yesterday he had not discussed the alleged payments to Johnson with his client but said that "as a personal matter, I can say that John Shorter wouldn't be involved in anything like that."

In another development, Clyburn resigned his position Monday as chairman of the board of Decisions Information Systems Corp., a local technology and consulting firm that held several District contracts or subcontracts, according to a statement released by company officials.

Shorter, who began serving his sentence at the federal prison camp at Maxwell Air Force Base in May, was convicted of one felony and four misdemeanor charges that he failed to pay $287,000 in taxes, interest and penalties. He has declined through prison officials to be interviewed.

A prominent criminal defense attorney who practiced law in the city for 23 years, he represented Joseph P. Yeldell, the former director of the D.C. Department of Human Services, who was acquitted in 1979 of bribery and conspiracy charges. He also represented well-known Washington lawyer William A. Borders, Jr., who was convicted in 1982 of conspiring to take a bribe.

In his own tax evasion case, Shorter said he was unable to pay his taxes because of his "overwhelming, uncontrollable impulse to use the funds for gambling," mostly at the race tracks, according to court papers.

Federal investigators uncovered evidence of the alleged payments to Johnson through wiretaps authorized as part of a 17-month undercover probe of D.C. government contracting that was disclosed May 22.

Johnson's refusal to testify three years ago stymied a federal grand jury looking into possible drug use by D.C. officials, including Barry, and others. Prosecutors intended to question her about a secretly taped conversation in which she said that she had sold cocaine to Barry on 20 to 30 occasions. Johnson was jailed for eight months on a contempt charge, then served four more months in a halfway house on cocaine charges. Barry has denied purchasing or using cocaine.

Johnson, 35, has met with prosecutors at least four times since May 22 and, as a result of her cooperation, is not expected to be charged. Sources have said she told authorities she received between $20,000 and $25,000 for her silence.

Clyburn and Littlejohn, political allies of the mayor, have denied paying Johnson any money in exchange for her refusal to testify.

The statement released Monday by officials of Decisions Information Systems Corp. said Clyburn had sold his interest in the company to other principals in the firm. Clyburn was the company's largest stockholder.

The statement said Clyburn told staff members that "because of publicity regarding my relationship with the mayor, I am a primary focus . . . of a broad-ranging investigation of contracting practices within the District of Columbia.

"My complete severance from ownership, direction or management of the company will provide me with opportunities . . . to continue to defend myself against baseless allegations of wrongdoing," the statement said.