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I can't decide between best or worst...
I managed to turn a completely won endgame into a draw.
This was a 1600 rated player, so I'm happy with the results.
I think that the book reply to 4. ... cxd4 is to temporarily gambit the Pawn and play Bd3 followed by something like O-O, B-f4-g3, Re1, Nbd2-b3 and then either work on recovering the Pawn or turn it into a real gambit by ignoring the d4-Pawn and going after Black's King.
Oh, you are right. It is mentioned in Moskalenko's book on the Winawer.