Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...d6/4. d4

= Ruy Lopez: Steinitz Defence = Not to be confused with the Modern Steinitz Defence.

4. d4
White threatens dxe4 since Black's knight is pinned. Thus, Black needs to play /4...Bd7/ (unpinning the knight) or 4...exd4.

Theory table
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 * 1. e4 e5 2. Nf3 Nc6 3. Bb5 d6 4. d4