WASHINGTON (AP) _ The Supreme Court today gave public employers added protection from lawsuits by workers fired for making statements later found to be constitutionally protected speech.

The court ruled in an Illinois case that public employers cannot be forced to pay damages if, at the time of the firing, they reasonably believed the worker simply was being insubordinate.

The decision yielded four separate opinions, and none commanded a majority.

But in the main opinion, Justice Sandra Day O'Connor wrote for herself and three other justices that employees can be fired for making insubordinate statements even if they made other statements that would be constitutionally protected free speech.

The court set aside a federal appeals court ruling that said Macomb, Ill., nurse Cheryl Churchill was entitled to a trial in her civil rights lawsuit against the public hospital that fired her.

She was fired by McDonough District Hospital in January 1987 after another nurse reported that Churchill had criticized her supervisor and said the obstetrics department was a bad place to work.

Hospital officials fired Churchill for insubordination. But she contended she actually had been criticizing her supervisors' policy of using nurses inexperienced in obstetrics to make up a staffing shortage in that department.

Churchill's lawsuit said those statements were constitutionally protected comments on a matter of public concern.

A federal judge dismissed her wrongful-firing lawsuit, but the 7th U.S. Circuit Court of Appeals reinstated it.

The appeals court said that if an employer fires a worker for speech later found to be protected under the Constitution, the employer can be held liable regardless of what it knew at the time of the firing.

Today's ruling set aside the appeals court decision, but allowed Churchill's lawsuit to go forward.

O'Connor wrote that as long as a public employer uses ''the care that a reasonable manager would use before making an employment decision,'' the employer cannot be held liable for firing someone who it believed made statements unprotected by free speech.

It is irrelevant whether the employee also made other statements that would be protected by free-speech guarantees, O'Connor said.

She added that the appeals court must restudy Churchill's lawsuit to determine why she really was fired. Churchill has raised a legitimate question about whether she actually was fired for her statements about the training procedures, O'Connor said.

Her opinion was joined by Chief Justice William H. Rehnquist and Justices David H. Souter and Ruth Bader Ginsburg.

Justice Antonin Scalia, writing for himself and Justices Anthony M. Kennedy and Clarence Thomas, also voted to set aside the appeals court ruling. But Scalia contended that O'Connor unnecessarily gave employees a new constitutional right to a pre-firing investigation by their employers.

Souter also wrote separately to agree with O'Connor's opinion.

Justices John Paul Stevens and Justice Harry A. Blackmun voted to uphold the appeals court ruling.

The Constitution's First Amendment ''demands that the government respect its employees' freedom to express their opinions on issues of public importance,'' Stevens wrote for the two. ''As long as that expression is not unduly disruptive, it simply may not provide the basis for discipline or termination.''

The case is Waters vs. Churchill, 92-1450.