Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. d4

4. d4

 * This aggressive move frees White's queenside bishop and adds pressure on the center. However, the e4-pawn is left undefended and that gives Black some options to establish a stable center.


 * 4...exd4 is Black's only credible answer.
 * 4...Nxe4 can be meet with 5. dxe5 when White threatens Bxf7+ or Qd5 attacking both the e4-knight and f7.
 * 4...Nxd4 with Nxe5 undermining the knight on d4.
 * 4...Bd6? blockades Black's d-pawn and allows White to take on e5.

Theory table
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 * 1. e4 e5 2. Nf3 Nc6 3. Bc4 Nf6 4.d4