1: Welcome to the boards
2: Congratulations on your trip
3: Sorry to say, I can't imagine any of us knowing the answer to your question.
If *I* had a 4-year old that really wanted to go on those rides (and he/she met any height requirement if there was one), I'd probably be willing to take him/her. But I also know his/her mother and I would probably have arguments galore about it. I took my 3-year-old godson on a spinning-tea-cup-esque ride at a local fair, and he went from excited to terrified in 4.3 seconds. You know your kid probably better than anyone else, and every day, some child's mettle is tested at every single attraction in the parks (I used to freak out like an animal in a spring trap during the fireworks. Too loud and imposing). All I can say is that you take your chances, let him know it's all make-believe, that Mickey Mouse wouldn't really do anything to scare him bad, and you yourself act excited but not imtimidated. "Whoa, that's cool!" Not "Oh, My God, that's scary!"