Well, here is what the movie has..
1. Violence
2. Very minimal swearing. I think there was one or two profane words.
3. Minimal sexual situations. I'm thinking more romance than swearing
4. Violence
I would think that this would be an OK movie for a 4 year old. I highly doubt that said 4 year old is going to understand the storyline as, with most Marvel movies, someone that young is going to fall short from their target demographics. But, if he is interested more in seeing Hulk smashing and Iron Man ironing than the developing storyline, this should be a treat. Keep in mind that this movie contains more violence than the first one.
Also, valuable information to any parent, this movie clocks in at just over 2 hours.