Puzzled by a federal judge's decision to hold a hearing on the Reagan administration's decision to dismiss the government's 13-year-old antitrust suit against International Business Machines Corp. four months after the case was officially dropped, both IBM and government officials are now weighing their options to see how--and if--they should block the hearing.

Although no decisions have yet been reached, government and industry sources indicated yesterday that both sides are looking at a variety of proposals, apparently ranging from appearing at the hearing to filing an appeal to block the hearing at a U.S. Appeals Court.

The reason: both sides contend that the case was legally dismissed by the government on Jan. 8 and that U.S. District Judge David N. Edelstein has no authority or reason to open a hearing into the dismissal.

Last Friday, in a highly unusual judicial procedure, Edelstein granted a special hearing to a Washington philanthropist and author, Philip M. Stern, who has charged that the suit was improperly dismissed because neither the court nor the public has had an opportunity to review the dismissal. Stern also charged that the dismissal was null and void because Assistant Attorney General William F. Baxter should have been disqualified from participating in the suit because of an earlier consulting relationship he had with IBM lawyers.

"There is no authorization or justification for these orders," IBM's senior vice president and general counsel Nicholas deB. Katzenbach declared. "The case is over; it no longer exists. I can find no explanation for Judge Edelstein's order."

Justice officials agreed, saying "our understanding is that the IBM case was dimissed on Jan. 8." According to one top Justice aide, "legally the case is gone, it doesn't exist."

The hearing was originally scheduled for April 8, but has been postponed until April 19.