I think it really depends on the child. My 4 y.o. is a huge Spider-Man fan and has seen Spider-Man 1 on DVD a number of times. My husband and I decided to go ahead and take the whole family (kids ages 4, 6, 11) and if anyone got scared, one of us (me no doubt!) would leave with the kid(s) who couldn't handle it. All 3 of them loved the movie and were not scared. No one even sat on my or DH's lap. No one came to sleep in our bed that night either which is a sure sign they must not have been scared.
I don't think this means that most (or even lots of) 4 year olds should see the movie. My son's best friend (also 4) gets scared watching the Little Mermaid on DVD. I think it's really an individual decision. And of course if a kid gets really scared or upset during any movie or is disruptive to other patrons, a parent should take them out.
So a 4 year old can see and enjoy this movie without being scared, but as always, your mileage may vary!
snoozn