So your proposed line was
1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Bg5 Bb4
I am not 100% sure but it seems that white gains an edge after e3 and then cxd5
let me investigate further
Ok I have done some further work on this and it seems like white just gains an extra move.
so say
5.e3 Nbd7 6.cxd5 exd5 7.Bd3
it feels a little better for white as later we can put the Knight on e2 rather than having already developed it to f3 as you would in the Ragozin lines.
If you want to play the ragozin, you have to be happy with the nid
Without addressing the core questions you have, I will say that black usually avoids playing 3...Nf6 because of 4.cxd5.
This is why after 1.d4 d5 2.c4 e6 3.Nc3 black usually chooses 3...Be7.
This is also why after 1.d4 d5 2.c4 e6 3.cxd5 exd5 4.Nc3 black usually chooses 4...c6.
Thanks for the responses. I have always had some interest in the Nimzo-Indian, but certain lines have always given me problems (um, cough! Saemisch and 4.Nf3). I'll have to re-examine the Saemisch before going into Ragozin Territory, which can be played against Kasparov's 4.Nf3 by answering 4...d5, but until then I'll have to continue with the Tarrasch, Slav, and Orthodox. I don't intend to completely throw those away, and would easily play them in a major tournament today, but when you play in 2 major tournaments a year, about 4 or 5 weekend tournaments a year, maybe 10 to 12 Saturday 3-Rounders a year, and the rest of your rated games are every Wednesday night at the club (i.e. 50 of my games each year), where you face players many times, you need more than just 1 weapon! 
In 2038 tournament games, I've got 10 opponents that I've played 25 or more times (87, 84, 76, 65, 46, 37, 26, 26, 25, and 25). Gotta be able to throw them off somehow!
I'll leave the King's Indian, Benoni, and Grunfeld for others. I've played the King's Indian for a while, and it's ok, and gotten King's Indian-esque positions from the Modern and Steintiz Variation of the Ruy Lopez (3...g6), but those tow lines do avoid certain problems entailed in the main KID.
As for the 3...Nf6 vs 3...Be7 argument, while 3...Be7 is a "legitimate" line, it's not "better" than 3...Nf6. I have absolutely NO FEAR WHATSOEVER of the Exchange Queen's Gambit Declined. It's one of two lines I have played for ever and a day with White, and fully understand what Black should do. First off, after 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.cxd5 exd5 5.Bg5, Black's best move order is 5...c6 as it creates specific nuances in both the 6.Qc2 and 6.e3 lines.
Of course, if both sides go main line Exchange all the way thru the opening moves, White's got 3 plans. The minority attack with b2-b4-b5, the central attack with f2-f3 and e3-e4, leading to either an IQP, Hanging Pawns, the Advanced Center (i.e. e4-e5), or dual IQPs (i.e. isolated pawns on d4[w] and d5[b]). The third possibility is castling queenside and the Kingside Pawn Storm.
I've actually had more trouble making the exchange work for White than defending against it as Black. My real preference as White is to simply play main line against the Tartakower or Lasker (i.e. early ...h6 lines) and against the Orthodox, play the Rubinstein (7.Qc2 instead of 7.Rc1).
@ThrillerFan My teacher (a top US Grandmaster trainer) gave me advice somewhat contrary to your experience. But one could argue that at anything below IM level that it doesn't matter. But here is a bullet point summary anyways:
The problem with the advice regarding 3...Be7 is that it obviates the chance to play the opening you want to play, the Ragozin.
If you want to play the Black side of the Ragozin and you like the Nimzo Indian, the obvious move order is 1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 and learn how to play against the Saemisch. There will always be openings that give you trouble.
If they play 4.Nf3 d5! is the Ragozin! This is how Kramnik, Aronian, Morozevich and many other GMs reach the Ragozin as Black.
Agreed with the esteemed Smyslov fan. Play through the Sämisch, learn it. At least you know going in that there is a solution, because it is not popular at the high level. There are ways for black to play.
I could be wrong, but I think he's basically talking the line in "Declining the Queen's Gambit" that came out last year. In essence, correct me if I'm wrong, but I think it's an early Bf5, like 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.cxd5 exd5 5.Bg5 c6 6.e3 Bf5 allowing for 7.Qf3 Bg6 8.Bxf6 Qxf6 9.Qxf6 gxf6, arguing that the wrecked pawns are hard to get it.
Avoiding it with 6.Qc2, Black has some line where he plays an early Nh5.
Again, correct me if I'm wrong, but I think that's what he's saying.
I won't have to worry about it anyway as this Ragozin stuff is just a work in progress. When I do decide to add the Nimzo-Indian back into my repertoire, this will be my response to 3.Nf3 or 4.Nf3, where if White exchanges, he's committed to Nf3, so whole different ball of wax.
For the time being, until that's taken care of, which may be a while as I'm currently working on expanding my repertoire as Black to 1.e4, adding additional lines of the Ruy. Until then, I'll probably continue to play the Tarrasch in serious competition, that along with the Modern (1...g6) that I've been playing occasionally as of late.
I have a question about move order in the Queen's Gambit Declined.
There is a well-known variation called the Ragozin Variation (ECO D38), which is thru move 4 by Black below, and a subvariation of that called the Vienna Variation (ECO D39), which is after each player's 5th move and White's 6th below
In today's day and age, nobody plays 4.Nf3. Everybody plays 4.Bg5. 4.Bg5 is D50 thru D69, with D50 being all the unusual replies by Black (i.e. Not 4...Nbd7, 4...Be7, etc).
The question is this: Are the Ragozin and Vienna only playable against 4.Nf3 and White has some move order trick in D50? Or can Black play 4...Bb4 or 4...dxc4 (intending ...Bb4 whenever White plays Nf3), steering the game to either a Ragozin or Vienna?