King's Gambit (fischer defense)

I am reading Joe Gallagher's "Winning with the King's Gambit" (good book). In chapter one, on the fischer defense, the first game goes 1.e4 e5 2.f4 ef 3.Nf3 d6 4.d4 g5 5.h4.

At this point, Gallagher states that "5. Bc4 is illogical; if white wanted to play this he should have played it on move 4, as then black would be obliged to play 4.h6.

I am looking at the position arising from 1.e4 e5 2.f4 ef 3.Nf3 d6 4.Bc4 and I can't seem to figure why black is "obliged" to play h6. I figure it must have to do with keeping the knight out of g5, but I'm not sure. Can anyone help me out?

After 1 e4 e5 2 f4 ef4 3 Nf3 d6 4 Bc4 Black is not obliged to play 4...h6.

It just seems like 4...h6 is the only move that has been played there according to my engine opening books.  Gallagher must have been trusting previous play.

4...Nf6, 4...Nc6, and 4...Bg4 all appear to be viable.

The point of the knigs gambit at all levels below 2000 is to create a sharp game. (which I do not perfer)