I'm not sure what I would play here, but I doubt that I would think of playing Black's move, 4...c5, leaving a backward Pawn on the d-file. On top of attacking the Knight, this interesting move prepares to bring the Queen out on the c-file and opens a more promising diagonal for the Bishop on e8.
The Knight on d4 has a number of retreat squares, but White chose 5.Ne2. The point appears to be that after 5...Qc6, the Knight defends the e-Pawn by 6.Nc3. After the further attack on the Pawn by 6...Nd6, White switched to an indirect defense with 7.h3, taking advantage of the open diagonal to Black's King. Black achieved a satisfactory game in the ensuing complications, when White decided to sacrifice the e-Pawn for an attack. The attack eventually fizzled, leaving White a piece down. For the complete game, see the PGN from Chess.com.
[Event "Let's Play!"]
[TimeControl "1 in 7 days"]
[Termination "alfloran won by resignation"]
[Variant "Chess 960"]
[FEN "rkqbnnbr/pppppppp/8/8/8/8/PPPPPPPP/RKQBNNBR w KQkq - 0 1"]
1.e4 e5 2.Nf3 f6 3.d4 exd4 4.Nxd4 c5 5.Ne2 Qc6 6.Nc3 Nd6 7.h3 Ng6 8.Bh2 Ba5 9.Nb5 Qxb5 10.Bxd6+ Bc7 11.Qd2 Qc6 12.Bxc7+ Kxc7 13.Ne3 Qxe4 14.Bf3 Qb4 15.c3 Qb6 16.O-O Rc8 17.b4 Ne5 18.Nd5+ Bxd5 19.Bxd5 Kb8 20.bxc5 Rxc5 21.Rfb1 Qd6 22.Rxb7+ Kc8 23.Qd4 Qxd5 24.Rxa7 Nc6 25.Ra8+ Nb8 0-1
How many players would think of a gambling move like 4...c5?