5.......Nxe4 6. Bc4, Be7 7. Nxd4, 0-0 8. Nb5, Bc5 9. 0-0, Nxf2 10. Qh5, Ne5 11. Nbxc7, d6 which all seems to be better for black.
5..........Be7 6. Bf4, d6 7. Nxd4, 0-0 8. Nb5, Nxd5 9. exd5, Ne5 10. Qd2, c6 11. Nc3, Bf5 which looks fine for black as well, although pretty even I think.
This may not be the best lines, as I don't play the 4 knights as black or white, but I think they are the two main lines to start from. unfortunately, as with any opening, there are numerous other lines at various points!!