For our last trip, my kids were 11 months, 3.5 and 6.5. Men in Black is my oldest's favorite Universal ride - and it was when he was 3.5 as well. But it's a 42" ride so I don't know if your 4 year old is tall enough. They also loved the Curious George Play area. ODS even enjoyed spiderman when he was barely 3 years old. Another favorite was the Jimmy Neutron ride, but I've heard that the ride is closed and being changed, so check up to see if the change is done. They liked Shrek too. Suess land was a hit as well. They like the Sinbad Show. My oldest loved the Terminator show.
I would not suggest Bluto's Bildge Rats unless you like getting soaked. My kids didn't want to go on that, so we didn't. But DH and I went on it before we had kids and soaked was an understatement. I've never been on a non-water park ride that I've ended up so wet on before! The Pterredon Flyers was another flop. It takes so long to load even during off days. The ride wasn't that exciting for them. The only other flop was The Simpsons for my middle child. She likes thrill rides and only Tower of Terror had scared her too much at that point. (Her little 30 lb body was way up off the seat on ToT, I don't blame her for being scared!) But after ToT, she wasn't frightened, she got over it. After the Simpson's, she was scared to go on any rides. So, use caution with the Simpson's, even with a bold child. Save it for the end!
*WWoHP was not open yet for our last trip, or I'm sure I'd have some comments on that. My kids are obsessed with Harry Potter. We will only have a one day ticket for our next trip and it does allow for two parks. We're sticking to IOA. On our last trip, we had two day park ticket and ended up spending only half a day at Universal but 1.5 days at IOA. We went at the end of Jan/start of Feb so it was the quiet season allowing us to get done with US quite quickly. We could have easily spent more time at IOA and that's without WWoHP being open.