Like other's have said, YOU are the best judge for your child. Don't let some people make you feel bad because you aren't "parenting" if you let your daredevil child go on a ride he is deemed tall enough. But also, don't let a child you know probably will get freaked out go because there are those of us that have children that young that LOVED it.
My son is 4. After watching the planning DVD religiously before our trip in January he HAD to go on TOT (he also wanted to do RnR but NO WAY!) I was scared about him going, but I know his temperement, and after he did wonderful on BTMR, and Splash (I know they're not identical, but the height restrictions are the same), I figured I'd go ahead and grant him his wish . . . . being fully prepared to deal with any negative consinquences. HE LOVED IT! And went on it over and over and over. To the point where mommy started to sit them out and he and Daddy did it again and again and again. (Mind you there were breaks in between several sets)
When we went again in April it's all he talked about wanting to do and sure enough he went multiple times.
Now our september trip is coming up and we are going to
MNSSHP and guess what he wants to be???? A TOT bellhop!!!!!
So, short answer is 4 too young? NO! Not if the child has the right temperement and meets the hieght restrictions.
I mean come on, there are some of us adults that well meet the height restrictions and can't handle the ride itself. lol.