The astronauts are four highly motivated Navy Seals in top physical condition.armed with the finest military assault weapons that the world's arsenals can provide, as well as being trained extensively in hand to hand combat and edged weapons techniques. The cavemen are four half starved, filthy, poorly clothed men, barely able to grasp the concept of verbal speech, and totally terrified of the strange sky people, armed with clubs and crudely made spears.
Or, the astronauts are four middle aged science professor types who haven't been outside of a lab since achieving puberty. They know absolutely nothing about fighting beyond what they remember from grade school, when the school bully used them as a warm up exercise before beating up the class nerds. The cavemen are four hulking Cromagnon warriors who bring down woolly mammoths on a regular basis, armed only with spears and stone knives. The smallest of the four could change the tires on an ATV without either a jack or lug wrench, and the biggest one can wrestle a 1500 pound cave bear into submission.
Take your pick.