In this position, 4. Bc4 is played the most. It continues White's development, and 4. Bc4 makes room for the king on f1 (which isn't a bad square, at least not any more than h4 is a bad square for the black bishop). E.g. after 4. Nc3, Black can play the nasty 4... Bh4+, forcing the king to move to e2. The calm 4. Be2 works as well, but I feel it is less in the spirit of the King's Gambit, which is an aggressive opening.
