Chess Opening Theory/1. f4/1...e5/2. fxe5/2...d6/3. exd6/3...Bxd6/4. Nf3

Bird's Opening
a b c d e f g h 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 a b c d e f g h
Position in Forsyth-Edwards Notation (FEN) rnbqk1nr/ppp2ppp/3b4/8/8/5N2/PPPPP1PP/RNBQKB1R b
Moves: 1. f4 e5 2. fxe5 d6 3. exd6 Bxd6 4. Nf3

4.Nf3

This is the most common move, and is also the best. The most common, and most aggressive reply, is the Lasker variation, 4...g5

Theory table

4	5	6
... g5	g3 g4	Nh4	+/=