The g forces are real. I have read this ride. You get into little rooms and are spun around by an arm like the ones nasa uses to test astronauts. However, you don't see the arm or spinning device, you are instead given a 'window' which looks like you are leaving the earth. It is extremely scary because it felt so realistic. I was literally terrified. I did not let my 5 year old ride two years ago and will not let him ride next month when we are going back to disney and he will be 7. You can certainly feel the g forces.
Good decision. From what I've read I think four is too young regardless of the height requirements. I want to keep my 12 & 16 year olds off when we go next month. Fat chance I'm sure. I see they need 4 people but they will have to do without me.