The fourth defendant to be tried in the Oct. 11, 1989, robbery and slayings at Stoney's restaurant in Prince George's County was convicted of first-degree murder and numerous other charges last night.

Derrel Thomas, 18, was accused of standing watch in front of the restaurant while two accomplices carried out the the $300 holdup, in which two Stoney's employees were shot to death and two other people were wounded, including an off-duty Prince George's police officer.

Although he did not enter the Clinton restaurant during the robbery, Thomas was convicted in Prince George's Circuit Court of two counts of first-degree murder and other charges under a legal theory known as accomplice liability. First-degree murder is punishable by up to life in prison.

Judge Vincent J. Femia scheduled sentencing for Jan. 23.

The two men who carried out the holdup, Damon Bowie, 20, and James H. Edmonds, 26, were convicted in separate trials in recent months. Edmonds was sentenced to 130 years in prison, and Bowie, the convicted gunman, was sentenced to die in Maryland's gas chamber.

Bowie's sister, Christian, 21, who drove the getaway vehicle, was convicted of two counts of first-degree murder and other charges last week and is to be sentenced Jan. 7. A fifth defendant who allegedly stayed with Christian Bowie in the getaway vehicle is awaiting trial.