Well, while those are the four best films imo, one definitely needs the others.
I, however, take that thought to an extreme. I have six copies of Alien, 5 copies of Aliens, 4 Copies of Predator, 3 copies each of Alien 3, Predator 2, and AVP, and 2 copies of Alien: Resurrection and AVPR.
Damn you, FOOOOOOOOOOOOOOOX!!!!!!!!