Its quite right that a 4 year old should be on Dumbo, Small World etc etc, and for precisely that reason, my 4 year old wont be going on it. I make the decision on the suitability of a ride for my daughter purely on enjoyment grounds though (as the height restrictions are out of my hands). As I said earlier, the Mission Space ride is really pretty tame....you dont go fast, there's nothing to scare you, you dont get shaken around, you do get to experience the feeling of weightlessness and the effects of gravity. My 4 year old wont be going on it next year (she will be 5 then), but for no reason of perceived safety unsuitability, she wont be on it simply because I dont feel she would really understand whats its all about, and would probably find it a bit boring. Im pretty certain the ride wouldnt produce the "again! again!" reactions that Woody Woodpecker, Goofy and Flying Unicorn obtained from her.
I have always thought Disney did a pretty good job at enforcing and assessing height restrictions, and to be honest, I trust their judgment completely.
Disney have been criticised in some quarters for pretty much immediately re-opening the ride. Aparantly 8.6 million riders have experienced it since it opened, without any kind of physical injury. I think they have made the right decision.....but then I think Ebay shouldnt have pulled the live8 ticket ad's, so what do I know ;-)