Black's g-pawn is attacked, and the only move that saves it is 4...g4.
4...f6 simply loses the pawn to 5.Nxg5! since 5...fxg5 6.Qh5+ Ke7 7.Qxg5+ Nf6 8.e5 is just carnage.
4...h6 is also no defence against 5.hxg5 pinning the h6-pawn.
4...gxh4 leaves Black with the world's most ridiculous pawn structure: doubled isolated pawns on f4, h4, f7 and h7. White doesn't even have to capture them right away; she can play something like 5.d4 if she wants to laugh at them a little longer.
4...Bh6 or 4...Be7 followed by 5.hxg5 Bxg5 6.d4! allows White to win the f-pawn back with the move g3 while remaining ahead in development.Rather than saving g-pawn, Black can counter-attack in the center with 4... d5 so that White's taking g-pawn is followed by Black's taking e-pawn: 5. Nxg5 h6 6. Nh3 dxe4.
== Theory table ==
For explanation of theory tables see theory table and for notation see algebraic notation.
1.e4 e5 2.f4 exf4 3.Nf3 g5 4. h4
When contributing to this Wikibook, please follow the Conventions for organization.
== References ==
Fundamental Chess Openings (2009). Paul van der Sterren. ISBN 1-906454-13-2.