Hello!
We were in the same position as you - debating between the BCV and Poly...we are currently booked at my first choice (VWL) because we have more room there, but DH is hinting we may call and change to Poly. We will also be there at the end of Sept, for DS's 4th birthday,
Why Poly over BC? We love the BC - but for our little guy we wanted to stay at a MK resort. The pool at BC is unbelievable, but the Poly has a fantastic pool and the convenience of the monorail and being able to just hop over to MK any time really weighs in for us more than SAB alone.
You'll be happy, no matter what. But I think a 4 year old celebrating a birthday would love a MK resort. A few other reasons? Wishes from the beach (or your room), the Electric Light parade every night, close proximity to Chef Mickeys (a must for our DS) and some other great character meals, and you can bop over to MK and back for a swim or rest any time. The walk from Epcot to BC isn't bad by any means, but it's a huge park and you could be in for quite a walk if DS (or you) are really tired.
Again - both gorgeous, wonderful resorts. No way you can make a bad choice.
And I think I just talked myself into switching to the Poly....Hmmmmm
(PS - I'll PM you with some ideas for the b'day and what we're doing just to get you thinking!)