Chess Opening Theory/1. e4/1...e5/2. Bc4/2...Nf6/3. d4/3...exd4/4. Nf3

4. Nf3
White has placed the knight on f3, supporting the idea of the pawn in e5 to expand in the centre and attack Black’s knight.

Theory table
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'''1. e4 e5 2. Bc4 Nf6 3. d4!? exd4! 4. Nf3'''