You can play either
1) 4 .. Nd4 a continuation that can either get very wild and exiciting or very dull and completly equal. So best used against someone who wants to win.
2) Personally I have a soft spot for the 4.. Bd6 system where you develop your kingside quickly and play Re8 creating a nice spot for your bishop on f8. It is a slower more positional game where white perhaps has slightly higher changes for and advantage but it is harder for white to get a draw
If I know someone is a Four Knights player as white I'd go for a Sicilian with 1...c5 (and hope for a Moscow variation, but the Najdorf and sometimes mainline or even 4...Qb6 Paulsen I'm okay with too) but if I don't know and find myself into it then I have a couple viable choices given by Chess Stars' Berlin Defence book and Marin's Beating the Open Games. I don't really like symmetry if I can help it but understand that black needs to deal with copying for a good while in many lines.