I saw it in 3D last night and it was fun. I would have preferred my first viewing to just be standard format though (I’ll be taking the kids this weekend although I have to kick around whether or not the 4 year old may be a little too young). Enjoy!!
I took four kids, the two oldest, boys aged 8 and 10 loved it. My 6 year old daughter thought it was fun but she just loves going to the movies and would enjoy anything. As regards my recently turned 5 year old daughter, she seemed to get a bit bored with it after the first 90 minutes.
So if your 4 year old is into SW he/she might enjoy it, if not you could probably leave them at home and save it for them for later.
On the whole absolutely child friendly (well except for an early scene involving a bloody hand print, which worried me a bit but overall fine).