I would not take a four year old child on such a ride.
It looks like Disney may have to up the realism by adding cast members to perform semi-real pre-flight physicals. At 3 Gs, I am surprised that this ride hasn't wiped out bus loads of seniors.
I just showed this story to my wife. We have an above-average height 4.5 year-old who rode Mission Space with me earlier this year. While the ride is a bit intense, she seemed unfazed. She asked me afterwards, "Dad, did we really go to Mars?"
There had to have been other medical conditions with this particular child.
I couldn't have kept my daughter off of it.
Depends on the kid. My son never liked this kind of stuff.