Four jets fr om an Air Force stunt team crashed today, one after another, into t he rugged Nevada desert and exploded in flames, killing all four pilo ts.

The four planes struck nose first into the sand while practicing a ''loop and tail'' maneuver, the Air Force said. ''The pilot farthest to the east hit the ground first and the other three followed within a tenth of a second, flying in formation,'' said Tom Sullivan of Boulder City, Nev., who was driving to a construction job in the area at the time.

''They did not collide with each other,'' he said. ''They didn't pull up fast enough, I couldn't believe they crashed. I saw the first one hit; there was a ball of flame, just like a napalm bomb.''

It was the worst accident in the history of the Thunderbirds, a precision military flying team, and its 15th fatal crash since the group was formed in 1953. A total of 18 pilots have been killed. Scattered Wreckage