With 4.d4, the Rosentreter Gambit, White indirectly defends against Black's threat of 4...g4. The most basic point is that White opens up their dark-squared bishop to recapture the f4 pawn. Most common by far is 4...g4 indeed, after which White has three very deadly options.

The most played and trickiest option is 5.Ne5, and after the ensuing 5...Qh4+ 6.g3 fxg3, 7.Qxg4 saves the day for White. The point is that Black's tempting move 7...g2+ is not best; after 8.Qxh4 gxh1=Q, Black has won White's rook, but after 9.Nc3, Black's position is extremely delicate; the engine evaluates this as equal, and it's extremely easy for Black to blunder. This line demonstrates very well how 4.d4 indirectly defends against 4...g4.

Another option is to play 5.Bxf4, the Testa Gambit; White sacrifices their f3 knight in similar fashion to the Muzio Gambit, looking for a deadly attack.

The third option is 5.Nc3, the Sorensen Gambit; once again, similar to the Testa and Muzio Gambits in sacrificing the f3 knight, but trying to demonstrate counterplay with the c3 knight instead of the dark-squared bishop.