A federal appeals court affirmed today the convictions of four of five Lorton Reformatory prisoners charged with taking four guards hostage in a 1983 incident, but ordered a new trial for one inmate.

A three-judge panel of the 4th U.S. Circuit Court of Appeals dismissed all appeals for Michael Lorick, Clinton Murchison, Willie Engram and Frank Joyner. They were found guilty of assault and kidnaping when they captured and held several guards hostage in an administration building of the District of Columbia prison in Fairfax County in February 1983.

But the court ordered a new trial for Sherman Dobson, who said District Judge Albert Bryan Jr. of Alexandria denied him the right to be his own attorney.

The government argued that when Dobson invited a court-appointed lawyer to participate in some pretrial work, he effectively waived his right to represent himself.

But the appeals court said the defendant again "expressly and unambigiously renewed his request to proceed pro se" at the start of his trial.

Dobson, 31, Lorick, 27, Murchison, 25, Engram, 42, and Joyner, 44, were serving life terms for a variety of convictions, including murder. They jumped the guards in a bathroom of the administration building.

Their plans were spoiled when one of the prisoners set off a mace container carried by the officer. The container could not be turned off, said Assistant U.S. Attorney Robert Cynkar. The prisoners stumbled into the building's hallway, where they were captured.