My kids would never leave without riding splash mountain (6&9...been the same since each was 3 lol). So, I just asked both and this is their list...from kids! :
1. Splash Mountain
2. 7DMT
3. Buzz lightyear space ranger spin
4. Big Thunder Mountain
5. Peter Pan's Flight/It's a Small World (haha..my kindergartener loves this one..sorry they were torn on the 5th one)
Both told me Space Mountain hurts their brain lol. I can't say I am a fan either, and it has a higher height requirement that may or may not suit your 4 year old. But please don't leave without doing Splash! The drop isn't scary and the storyline in the ride is seriously the best!