According to Pinski, Black's main responses are 4...Bc5 and 4...d5, both of which are reckoned to equalize for Black.
Cf1-b5** 4…Cf8-b4 — со сложной маневренной борьбой** 4…Kc6-d4 — контратака, впервые применённая Ф.Маршаллом.