Isn't 6...c5 the main move here? I cannot say for certain as while I have played 1...d5 occasionally, I mostly avoid this stuff. Spent most of my life playing the Kings Indian, Nimzo Indian, and Dutch. But memory seems to recall that 6...c5 is the main line here, is it not?
How to deal with the Harrwitz attack in the QGD?
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4..nbd7 looks interesting. 5 bf4 dc and nd5 seems playable. 5 c5 b6 looks better for black than the line where the bishop already stood on f4, as now 6 b4 a5 and white can't play a3, so 7 c6 but 7..ab sacrificing the knight for 2 pawns, or 6 c6 nb8 when white seems to keep the pawn with some nice tactics but black is not worse on the resulting positions. At least it looked that way when I analysed.
Worse "in."
Just consider the c7 square, the diagonal and a Rook on c1.
4....Nb-d7 is an interesting move order, which hasn't been covered in books much. If play the semi-tarrasch, when white tries for the exchange variation 1.d4 d5 2c4 e6 3Nc3 Nf6 4pxp Nxd5, then can also play 4Nf3 c5 with little extra work. White trying 4Bf4 to reach the same position doesn't really work after 4...c5 5pxp Bxp 6Nf3 Nb-d7 or 5e3 pxp 6pxp Be7.
This is by far the most testing variation to face as black.