Instead of 4.Nf3, the move 4.e3 leads after 4...e6 5.Nf3 to the Semi-Slav, after 4...a6 5.Nf3 to the ...a6 variations that we saw earlier, and after 4...g6 5.Nf3 to the Gruenfeld. The continuation 4.cxd5 cxd5, followed by 5.Nf3 is the Exchange Variation; here White has the option, however, of 5.Bf4, to delay Ng1-f3 for as long as possible.
Instead of 3...Nf6, Black can play 3...e6. After 4.Nf3 Nf6, this also leads to familiar variations: 5.e3 is again the Semi-Slav, and 5.Bg5 the Queen's Gambit Declined.
One important, independent variation is 3...e5, which is tricky for both sides. If 4.dxe5, Black has 4...d4 5.Ne4 Qa5+. White's best is probably 4.cxd5 cxd5 5.Nf3, forcing Black to protect the prematurely extended center.
Another independent idea is 3...dxc4 4.e4 e5 5.Nf3 (of course not 5.Bxc4? exd4) which looks like a Queen's Gambit Accepted, where Black has played ...c6.
