You might consider this one as well: http://www.telescope.com/control/pro...oduct_id=09804

Or even a 6" dob such as this: http://www.telescope.com/control/pro...oduct_id=09185 - it'll allow for some 'growing room', although the 4.5" might be nice; at that size, your 4 year old will soon be able to set it up himself - thus given him a sense of ownership in the scope.

Scroll down on this page: http://scopereviews.com/page1s.html there is a review on the 4.5" XT dob.