4.Nf3!
This is most accurate, threatening 5.e4. After 4...Bf5, White achieves a large advantage with 5.Qb3 e6 (5...Nc6 6.Nbd2! Nb6 7.e4 Bg6 8.d5 is very strong) 6.Nc3 (avoiding the complications of 6.Qxb7 Nd7; 6.Nbd2 is also good) 6...Nc6 7.e4 Nxc3 8.exf5 Nd5 9.a3 (avoiding 9.Qxb7 Bb4+) Qd6 10.Qxb7 Rb8 11.Qa6 Be7 12.Bb5 Rb6 13.Bxc6 Rxc6 14.Qd3 exf5 15.0-0 0-0 16.Qxf5, as in Lipnicky–Bondarevsky, USSR championship 1951. White also achieves a "pleasant advantage" with 5.Nbd2 Nf6 6.Qb3 Qc8 7.g3 (Benjamin).
Read more about this topic: Marshall Defense