In move 4, Bd2 is not the best reply. e3 would be going a tempo ahead because you would set up a nice chain and center. When he does take the knight, the b pawn recaptures which means the black bishop is free, and the board looks like a queen's gambit. You get doubled pawns in the c file- which means double chances to attack either a b pawn or a d pawn. Just a little note.