totally depends on you... If you like NYC, then sao paulo is good for weeks on end... Lots of amazing restaurants, strong dollar = deals...

Plenty of museums, STELLAR nightlife, lots of shopping, cute neighborhoods... Look into Jardins, villa madalena, and Liberdade to start.

If you are not a city person, get a car and drive towards santos to the beaches of guaruja. Quick easy drive (once you get out of insane sao paulo).

head to the mountains in Campos do jordão. A nice mountain town, with a swiss heritage.

Rio is only 4.5 hour drive, or a quick cheap flight, if you want easy beach / tour destination

4 days is actually pretty short for my taste I've been to brazil over 60 times, and it never gets old...