Personally, I've been to Busch Gardens twice and felt like I've been there one time too many (sometimes even twice too many). It is mostly extreme roller coasters (Sheikra) and not much of the shows (although there are some). And, most likely, your 4 year old would not be able to go on most of the rides.
We finished the park in 3 hours and were ever-so-glad we drove there so we could leave and spend our day shopping in Tampa. If you are looking for a diversion from Orlando, and really enjoy roller coasters, then I would consider it.... but in my experience, my friend and I loved roller coasters and hated BG.
I would recommend a day trip around MK.