With Nc6, Bg4 and 0-0 the simplest plans. White can try g3 to break the pawn chain but there are obvious risks. As a general rule of thumb if you accept the gambit try to hold the pawn as if white can play Bxf4 he is a happy bunny.
In the case of 4.h4 you can try:
Of course there are many other lines as well!
Is there a simple line from black view against kings gambit?