Every child is different, so I am sure there are some who could ride this ride and not have a problem.
However, if it were me, I wouldn't take my 4 year old for 3 key reasons
1) It is a simple lap bar mechanism which will make a person not feel like they are securely fastened in... especially if they are younger
2) My kids always struggled with darkness in rides when they were young. The pitch darkness of the ride might be a little much for a young child.
3) The ride is still very jerky, and it is a little unnerving even for me because with that lap bar and jerking around in the dark in a narrow car with a single chair you can feel at times you are coming out of it.
Maybe your child could handle it, but I personally wouldn't take the chance.
On a personal note, we were at DLC the year they opened ToT. Our kids were fairly young and we didn't think they would enjoy it. Someone at the hotel said, "Oh we took our little boy and he absolutely loved it." Our kids had always been pretty brave so we figured why not, what can it hurt. We took them on and it absolutely terrified my youngest. She was small and the bar across her lap didn't keep her firm in the car. It literally ruined the rest of her day. To this day I can't get her to try it again. Sometimes it is just wiser to wait until you are truly confident your kids are good with it
Hope you have a Magical Trip!
