As others have said Poseidon's Fury isn't really a stunt show. Personally, I love Poseidon and can't stand Sinbad as I find it boring. However, if your 4 year old gets scared on POC, I would not suggest Poseidon.
Spoilers below
Poseidon is a walking tour that goes horribly wrong in an overly dramatic comical way. You are told you are there for an archialogical tour, but then you become trapped in the dark with no way out and an evil bad guy after you. The guide panics and proclaims you are all going to die. I've seen plenty of small kids in tears at this point. After that it gets better as eventually you are saved and do escape. Nothing jumps out at you or anything, but it does go completely pitch black a few times. Since it is more of an interactive show where the audience is a part of the show in that the guide interacts with them as a part of the story, it can be hard for some kids to understand that it is all pretend. Whereas a stage show is easier for them to differentiate between make believe and reality. If your 4 year old can understand it is all pretend and acting, it will be great fun. If he thinks it is real, it could be terrifying.