One of four helicopters carrying President Reagan's entourage last week flew within 500 feet of a private plane after a communications mix-up briefly kept the helicopter's pilot from contacting air traffic controllers, officials said yesterday.

The incident, which did not directly involve the helicopter carrying Reagan from Manhattan to Newark International Airport on Sept. 21, has prompted a review by the Federal Aviation Administration of its air traffic handling of the president's helicopters.

FAA and White House spokesmen acknowledged the communications mix-up but denied that the escort helicopter and the private twin-engine plane were in danger of colliding.

"At no time was safety compromised . . . nor was the president's helicopter, Marine One, involved in the incident," FAA spokesman Bob Buckhorn said. "There were no mid-air collision reports filed. It was a perfectly safe operation."

Buckhorn said computer tapes were being reviewed to determine exactly how close the two craft actually came.

Rep. Guy V. Molinari (R-N.Y.) said FAA officials have told him that the escort helicopter and the plane came within 400 feet to 500 feet of each other as both were making an approach into the Newark airport.

Officials said the problem occurred when controllers in the Newark tower failed to inform the controllers relieving them that the presidential helicopters, which had flown from Newark to Manhattan earlier in the day, had been assigned a special radio frequency.

When the helicopters lifted off from Manhattan, the pilot of the lead escort helicopter could not raise the control tower through the special frequency and finally switched to the regular frequency. The controller on duty expected the helicopters to contact the tower on the normal frequency.

As a result of the mix-up, "we're reviewing air traffic control procedures used in handling the president's helicopters," Buckhorn said.

The helicopter was carrying Secret Service and White House staffers, officials said. Marine One, which carried Reagan, was described as about five miles from the private plane.

The president was in New York City for a speech at the United Nations. The four helicopters flew the entourage from Newark, where Air Force One had landed, to Manhattan and back.

Last August, a private plane flew within 150 feet of Marine One in restricted airspace as Reagan neared his ranch north of Santa Barbara, Calif.