Chess Opening Theory/1. e4/1...a6/2. d4/2...b5/3. c4/3...Bb7/4. Nc3

= St. George Defence/Three Pawns Attack = The most intuitive move is probably the best one : /4...b4/ threatens the newly developed knight.

Theory table
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1.e4 a6 2.d4 b5 3.c4 Bb7 4. Nc3