How to counter 3. Nf3 f5 in the King's Gambit Declined I've read about many of the basic variations of the KGD, e.g. these but none of of them mention the following from Black. This suggests to me that this move may be a mistake from black that I could take advantage of, but I see nowhere good for white to go from here.

What should white play next?

[fen "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

1. e4 e5 2.f4 d6 3. Nf3 f5
3...f5 is not a very good move, but it's not a terrible one; there's no immediate refutation. White's best response (verified with the chess program Houdini) is probably to deal with the pressure on his e-pawn by playing 4.exf5 Bxf5 (4.d3 would be passive; 4.Nc3 fxe4 5.Nxe4 d5 gives Black a nice center), then continue to develop, say with 5.Nc3 (covering e4 so that fxe5 is a real threat; if 5.fxe5 dxe5 6.Nxe5 now then 6...Qh4+ 7.g3 Qe4+ double-attacking the knight and rook). White's position is more pleasant but Black is certainly not lost.