DD and DS really enjoyed our Pal Mickey. We bought him as I'm not sure a 4 yr old would like to hand him back at the end of the trip. If you are unsure you can always rent and if you decide to buy you can apply your rental to the purchase (they give you a new one, I believe)
It would be kind of fun to have him prior, to learn the games and hear the jokes. It'll probably really surprise your DS once you get to WDW and PM starts saying all kinds of different things.