The dramatic announcement that the jurors in the O. J. Simpson murder trial had reached a verdict less than four hours into deliberations startled and confounded virtually all of the legal experts following the case, most of whom had confidently predicted that the denouement of this extraordinary trial would take a week or more.

Most experts had predicted that the jurors, having been sequestered for almost nine months, longer than any panel in California history, would take a few days simply to vent their emotions out of an enormous sense of relief that they could finally do so. Until today, they had been forbidden to discuss among themselves what they had seen and heard about the killings of Nicole Brown Simpson and Ronald L. Goldman.

There was also a sense, given the gravity of the double homicide charges against Mr. Simpson and the defense's contention that the police and prosecutors had acted hastily against him, that the jurors would be inclined to dissect a good bulk of the evidence before taking any vote on the defendant's guilt or innocence. Mr. Simpson's chief lawyer, Johnnie L. Cochran Jr., made exactly that point during closing arguments last week, imploring the jurors to carefully examine the 1,105 pieces of evidence and the more than 45,000 pages of trial transcript and not "rush to judgment."

But the jurors had apparently had enough. In a stunningly swift move that shocked the courtroom and a nation still debating the finer points of closing arguments last week, the jurors filed back into the hushed courtroom at about 3 P.M. to confirm that they had reached a verdict. Judge Lance A. Ito said it would be announced on Tuesday at 10 A.M.