Fair movie, watchable, but it's popular for its technological gloss, not for a great story. The story is ok, perhaps a B-, so your 4 or 5 yr old would be better served first by something with quality (like Charlotte's Web, Finding Nemo (although that one is frentic and better after age 5), and very high quality movies (only a few) like that recent Japanese one with the little people or some Disney movies). Of course, 4 and 5 yr olds don't especially need feature films. They do fine without them. Ideally, we'd let them play and even use their own imagination to make up something, etc.
About the violence: our 5 yr old, who has seen a number of films, was distressed by the angry shooting of one of the main characters, so we had to turn it off, and later resume after the violent part.