Queen's Gambit Accepted with black trying to defend the c file pawn with 4. ... Bd7 In the following opening:

[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4


There is a well-known trap if Black tries to defend the c file pawn with:

[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4 c6


However I got the following played against me:

[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4 Bd7


How should White respond?
[fen ""]

1.d4 d5 2.c4 dxc4 3.e3 b5? 4. a4! Bd7? 5.axb5! Bxb5 6.Bxc4! Bxc4 7.Qxa4+ 


and White gets back the bishop with the advantage ( no matter what Black plays Qxc4 is White's next move ).

Hopefully this answer helped.

Best regards.