Blue to play.
Solution: Here the cliché, “The best form of defence is attack," is accurate: 4….a6-b5 is shown up as the first blunder of the game, if Blue plays 5.g3-c3. There is no defence against the threat of 6.e1-a5, and Blue wins on the next move. (If 5….b6-b4-d2, then either 6.f2-d4-b6 wins or g2-e2-c2-c4-a6 wins; if 5…c5-a5, then 6.e3-a3-a7 wins.)