4...bxc4 is the best line. You are right in saying it is very common in blitz here. Exchanging on c4 immediately gives you all the initiative because it opens the b-file for your rook. A lot of times you can exchange sac it for the b2 bishop with delicious results.
Christian Manley vs. Jan rosenburg, 2001 is a good example of the kinds of trouble White is in with 4. b3:
http://www.chessgames.com/perl/chessgame?gid=1173840
bxc4 is the main line to go instead of 4.g6 I searched this in game explorer, there's not much though of the opening, can't really guide you any further as there is not many more games.