2 classics, 1 very good and 1 Okay film:

This is a superb set of films,

Alien is an all time classic sci-fi movie, tense and atmospheric, it is worth seeing for the chestburster scene alone.

Aliens is yet another classic and one of the best sequals of all time, an action packed edge of your monster movie truly kick ass.

Alien 3 isn't a classic movie but it is definitely a worthy addition to the series.

Alien Resurrection is pretty ropy there really was no need for the continuation of the Alien series, after Alien 3 wrapped everything up so well, but this is just an example of the Hollywood money machine milking a franchise for all its worth. This is by no means a bad movie its worth seeing at least once.

I can't reccommend this set of movies enough rent them, then go and buy them and watch them again an again.