I spent a full day in Yangon and the only one place I would unreservedly recommend is Shewdagon Pagoda, which is best visited during day and night. Since I was on a escorted tour, I didn't take the Circular train, which by all accounts is an experience. Just walking around the city area looking at the street hawkers, old colonial buildings, streetside bookshops and people is very enjoyable too.
I would think four days is probably too much. If you can, book an early flight out to Bagan about half an hour away. It's a whole different world there and the plains of Bagan with hundreds of pagodas is one of the best sights I have ever seen.
