Why is the Ragozin 4... Bb4 not even considered against 4. Bg5? The Ragozin variation of the Queen's Gambit starts with 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Nf3 Bb4:

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1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Nf3 Bb4


It seems that the Bb4 move is only sound in that position, and not if white played 4.Bg5 instead of 4.Nf3. 

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1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Bg5 (4.Nf3 Bb4) Bb4


This line is, in my experience, not even mentioned in common opening books, as if it were "obviously" wrong... I'm not sure why. If this  is bad for black, which would be white's best answer?
I'm not an expert but my guess is black doesn't want white to develop his g1 knight to e2 in a variation he commits his bishop to b4. 

For example after 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Nf3 Bb4 5.Bg5 h6 6.Bxf6 Qxf6 7.e3, wouldn't you be more comfortable with your f3 knight on e2? You can get that in the other variation!

1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Bg5 Bb4!? 5.e3 keeps the g1 knight flexible. Now if
5...h6 6.Bxf6 Qxf6 7.Nge2 and white slightly improved his position. That is if you agree the knight is better on e2. There are more than hundred master games played in my database with 4.Bg5 Bb4 so I don't believe it's dubious, just slightly inferior.