A U.S. magistrate yesterday revoked the bonds of four defendants in a large-scale drug distribution case. All four are accused by the government of plotting to pay $25,000 to have the key witness against them murdered.

Ronald Hinton, 36; Thomas Watts, 37, and his brother Clarence Watts Jr., 36; Joseph Lester Brown, 41, and a fifth man, Marvin Lee Cobb, 30, were arrested on June 6 and described by law-enforcement officials as long-time suppliers of the drug, Preludin, known on the street as "Bam."

All five men are charged with drug-law violations, but Cobb is not implicated in the alleged murder plot, authorities said. Defense attorneys said they would appeal the bond-revocation. Cobb is free on bond.

Magistrate Lawrence S. Margolis initially ordered Hinton, the Watts brothers and Brown held without bond last Friday after hearing testimony that a government informant, identified only as S-1, was told by one of the defendants of the murder-for-hire scheme. Margolis postponed a final decision on the bond question until the defense could present its evidence yesterday.

Hinton's lawyer, Sol Z. Rosen, identified his client yesterday as the man the government contends made that statement to the informant. Hinton took the witness stand yesterday and denied that he or any of his three co-defendants had plotted or even discussed harming the witness.

The alleged murder scheme was said to be directed against John Henry (Sunny) Tate, the key prosecution witness who was understood to be closely associated with the defendants and who is now in protective custody.

Last Friday, D. C. police Lt. Ronald Harvey, who supervised the 10-month investigation that led to the arrests, testified tht his informant, S-1, had learned that the four defendants had pooled their resources and collected $25,000, which they intended to pay out for Tate's murder. Harvey said the contract was for delivery of Tate's "head."