It looks as though your options are 5.c3, 5.h3, and 5.d3, popularity being in that order. I don't play the Italian and when I did, it was the Slow Italian, so not sure what your intentions are with 4.O-O vs the more normal 4.c3, but no matter what you play, you need to understand all of your opponent's possibilities, not just your favorite line.
I love the Raptor Variation of the Trompowsky (1.d4 Nf6 2.Bg5 Ne4 3.h4), but I still must know answers to 2...d5, 2...c5, 2...e6, 2...g6, 1...d5, 1...e6, 1...g6, 1...f5, 1...d6, 1...c5, 1...c6, 1...b6, 1...b5, etc.
For you, it's 4...d6, 3...Nf6, 3...Be7, 2...Nf6 (what I play), 2...d6, 2...d5, 2...f5, 1...e6 (also what I play), 1...c6, 1...c5, 1...d6, 1...g6, 1...Nf6, etc.
So I play an aggressive line in the Italian where I castle early for a quick d4 but it only works with 4 … Nf6. What about 4… d6 ?? Its an extremely common move and takes me out of my comfort zone by switching the move order so I dont get what I want. What to do here?