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

4.d4
This is a common move in the From's Gambit for White, defending e5 and attacking g5 at the same time. 4.h3 was weakening, but also playable. However, it was not in the spirit of the From's Gambit.

Black can now continue his plan with 4...g4.