Former Northern Virginia school psychologist Arthur S. Pomerantz pleaded guilty in Fairfax County Circuit Court yesterday to four counts of engaging in sex with teen-age boys and was found guilty of a fifth charge.

Pomerantz, 47, whose case forced a tightening of the county's school personnel practices, was ordered jailed by a judge who said he is likely to be sent to prison.

"They're very serious charges," Judge Johanna L. Fitzpatrick told Pomerantz. "I know you have no prior record but the chances are you will be incarcerated . . . so you might as well start now."

The psychologist faces up to five years in Virginia prison on each of the five counts. In return for his plea, prosecutors agreed not to press any other similar charges against him.

His arrest Aug. 16 prompted an outcry when it was learned that his school coworkers had complained about his interest in young boys several years earlier after Prince William County police investigated him on similar charges. Pomerantz, who previously lived in Woodbridge, worked for the Fairfax County schools from 1965 until 1984, when he resigned under pressure.

Fairfax School Superintendent Robert R. Spillane conceded the schools should have been quicker to act against Pomerantz and promised to personally check into police complaints against school employes and to make more detailed background checks of prospective workers.

All five of the boys involved with Pomerantz were students in the county schools, but lawyers said none met him in the schools.

Pomerantz' lawyer, Mark E. Sharp, said yesterday his client is attempting to be admitted to a Johns Hopkins University clinic in Baltimore that treats pedophilia and other sexual abnormalities, but has been unable to gain admission because of its waiting list.

Fitzpatrick scheduled his next court appearance for Friday. Pomerantz has been free on bond, living with his mother at a Maryland retirement community, since last fall.

Pomerantz -- balding, wearing a gray suit, his hands clasped in front of him -- told the judge he wanted to be treated at Johns Hopkins for "the difficulties that I'm having . . . . I'm looking forward to participating in the program."

Commonwealth's Attorney Corinne J. Magee did not oppose Sharp's motion to involve the Hopkins clinic in evaluating Pomerantz, but said after the hearing, "He has had counseling before. I want to see penitentiary time."

Pomerantz pleaded guilty in a barely audible voice to four counts of sodomy, and, on a fifth charge of indecent liberties, agreed that the evidence was so overwhelming against him that he would likely be found guilty even though he did not admit to the charge. Fitzpatrick found him guilty on that charge as well.

The incidents took place on March 1, 1982; April 30, 1983; June 5, 1985, and June 11, 1985, and involved three 13-year-old boys and one 14-year-old. The June 11 incident involved one of the same youths with whom Pomerantz's Falls Church roommate, Edward Buss, pleaded guilty last week to having sex with a day later. Buss pleaded guilty last week to a second sexual assault charge.

According to testimony by police investigator Daniel Golehart, Pomerantz met two of the youths through Buss, a third youth through one of his earlier victims, and a fourth youth at a county recreation center. One boy was the son of a friend of Buss' who allowed her son to stay with him.