The head of the Justice Department's criminal division said last night that failure to explain a three-to-four month delay in a grand jury investigation earlier this year probably contributed to a grand jury foreman's charge that the department was covering up to protect aides to the president.

Philip B. Heymann told reporters in his office that it was "discourteous and a mistake" not to tell the grand jurors that the FBI was interviewing witnesses about a scheme by fugitive financier Robert Vesco to approach aides to President Carter in an effort to fix his legal problems.

Ralph E. Ulmer, foreman of the Vesco grand jury, tried to resign last month, claiming the department had withheld information and delayed the grand jury process. His allegation seemed to gain some credence in light of reports that Heymann had told grand jurors he doubted the truthfulness of White House aide Richard Harden.

A Vesco emissary had paid $10,000 to W. Spencer Lee IV, an Albany Ga., lawyer, to approach top Carter aide Hamilton Jordan, his boyhood friend, in early 1977. Instead, Lee claims, he saw only Harden, who talked him into dropping the plan to help Vesco. The financier had fled the country after being indicted on charges of stealing a fortune from stockholders.

Still unexplained is why Harden told Carter about Lee's approach and why Carter then wrote a cryptic note to then Attorney General Griffin B. Bell, telling him to see Lee.

Heymann was reluctant to answer questions about the Vesco case or the current FBI investigation of Jordan, who was recently accused of using cocaine at New YorkS Studio 54 disco last year by its indicted owners.

The criminal division chief repeated earlier remarks about his concerns with the special prosecutor law that triggered the Jordan investigation. He said he believed the new Ethics in Government Act should be amended but said it would be "too tough a thing to tackle in an election year, and too tough to tackle within a year of enactment of the bill."

He also said he was convinced that there could be no coverup of any major investigation in the department.