Thank you for your reply Ashley.
I really wouldn't mind if it was a case of a slight fluctuation in height but if I make the height acceptable on the 3gs (i.e. so he can make the jump), the iPhone 4s is literally jumping out of the screen, probably double the height. I'm not sure if I'd go as far as to say this was a bug, but it does make platform games near unplayable at the moment on mobile platforms.
Are there any work arounds or is this possible to fix?
(P.S. I really appreciate you taking the time to respond to us)