A four year old might enjoy the Virtual Jungle Cruise. Also, if they are interested in cars I reckon "helping" an adult to steer on any of the racing games could be a winner!
Sorry, but I have to disagree on this one, last year my boys were 7 & 4 and they had a ball, they love video games anyway, but they had a blast and they were actually upset this year that we didn't go. I must say though that last year when we went, DQ was included in the Park Hopper option, now you have to pay extra for it and I don't know that I would do that.