Chess Opening Theory/1. f4/1...e5/2. fxe5/2...Nc6/3. Nf3/3...g5/4. d4/4...g4

4...g4
This move continues the plan of Black's 3...g5. He is kicking the f3-knight away. Houdini recommends the passive-looking Ng1 d6 e4!, giving back the pawn. After dxe5 d5 Nce7 Nc3 f5 (Otherwise white gets a bind on the f-file), Bg5 h6 Bh4 Bg7 Bb5+ Kf7 Bc4 Qd6 and both exf5 and Bb5 are good for white.