A 19-year-old Northern Virginia youth charged with involuntary man-slaughter in connection with a spectacular two-car crash last November testified yesterday he had drunk the equivalent of five beers in the six hours prior to the accident in which four people died.

Jonathan Peter Reinemer of Falls Church told a Fairfax County Circuit Court judge he had just left an Annandale beer party and was driving at a speed "somewhere under 50 miles an hour" when he passed a friend's car on a curved stretch of Annandale Road, swerved across the center lane and slammed broadside into an oncoming car.

All three of Reinemer's passengers and the driver of the oncoming car were killed in the Nov. 9 accident, one of the worst in Northern Virginia in recent years. The accident received widespread publicity after a misidentification of two of the female victims resulted in the cremation of one body by a family who thought their daughter was dead when actually she was lying unconscious in the intensive care unit of Fairfax Hospital.

If convicted, Reinemer, who has pleaded innocent and is free on $25,000 bond, faces a maximum sentence of 20 years in prison.

Fairfax assistant prosecutor Steven Merril tried to convince Circuit Court Judge F. Bruce Bach yesterday that Reinemer, who suffered a fractured pelvis in the accident, was drunk and that his intoxication caused the crash.

Defense attorney Robert Hall, conceding that Reinemer exceeded the posted 35 mile-an-hour speed limit, argued that Reinemer "was not intoxicated" and the accident was the result of a "mechanical failure."

"The problem has been that the result is so terrible," said Hall as several relatives of the crash victims listened in the courtroom. "There is no clinical evidence of intoxication," he said, accusing Merril of "making a man-slaughter case out of a reckless driving case."

Hall presented no testimony yesterday specifying the nature of the alleged mechanical failure. A Fairfax police officer who investigated the accident said he discovered no defect in Reinemer's 1974 Chevy Nova that would have caused the crash.

Hall's statement that Reinemer was not legally drunk was contradicted by State medical pathologist William F. Enos, who testified that a sample of Reinemer's blood analyzed six hours after the crash showed that he was well beyond the point at which motorists are considered legally intoxicated. Enos said Reinemer's blood alcohol content was probably between .18 and .20 of one percent at the time of the crash and was at .12 six hours after the accident. In Virginia, a driver whose blood alcohol content exceeds .10 is considered intoxicated.

The trial is scheduled to conclude today.