Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. Nd2/3...c5/4. Ngf3

With 4. Ngf3, White keeps the tension in the center for a little bit longer, and defends the d4 pawn. This often transposes to positions in the 4. exd5 lines, though a few independent lines do exist.

4...cxd4 is the most played move, and the move that players who take back with the Queen after 4. exd5 usually play. White can transpose to the main line Chistyakov Defence with 5. exd5 Qxd5, though an independent line with 5. Nxd4 also exists, resulting in slightly different positions compared to the normal Chistyakov Defence.

4...Nc6 is the second most played move, and the moved that players who take back with the pawn after 4. exd5 usually play. White usually transposes to the Main line by playing 5. exd5, with black almost always responding with 5...exd5.

4...Nf6 is the third most played move, and can transpose to the Closed Tarrasch after 5. e5, though White also has the option to remain in the Open Tarrasch with 5. exd5.

4...a6 is a rare option, but often arises after Black plays 3...a6 in the Tarrasch. Usually White responds with 5. exd5 or 5. dxc5.