Spiderman is one of the BEST rides and completely suitable for a 4 year old. It's a 3D simulation ride where you actually feel like you are going through the comic book.
Directly from the Universal Website:
"Your Spider-Senses Will Be Tingling. Put on your 3-D glasses for an eye-popping, pulse-pounding, first-of-its-kind 3-D Spider-Man thrill ride.
Careening through the streets and swinging high above the city, you'll see, hear, and actually FEEL the action of Spider-Man's most amazing adventure ever."
I LOVE this ride, and until you actually ride it, I find it extremely hard to describe. But again, I find that it is completely suitable for kids and in fact and not scary at all. Caution though - if you have problems with simulation type of rides, you may have a problem with this one. I have to take motion sickness pills for this one!