Basic QGD Question -- White's move 4

I read, in two separate sources, within the space of a half hour, two different ideas on move 4 of your standard QGD

I see in opening explorer, 4 Nf6 is the most popular move, by far.  I read elsewhere a comment that said: 4 Nf6 is important because you do not know if your opponent will the semi-slav, so playing Bg5 is not recommended until move 5.  

On the other hand, in my "Starting Out: Queen's Gambit Declined" the author, in the intro, describes 4 Bg5 as "a strong attacking move which keeps up the attempt to overthrow the d5 point".  Now I realize that if white goes to semi-slav with 4...c6 that will help defend d5.  Nevertheless, I don't know enough to understand what's wrong with white's bishop being on g5 if black transposes to a semi-slav.

(And, in any event, isn't it a good thing for white to get the bishop out before white plays e3?)

So . . . I'd love to hear discussion of the pros and cons of white's fourth move from the above diagram.

Thanks!

Bg5 is extremely common in both the QGD and semi slav... so the only reason to recommend waiting is that your repertoire doesn't include it for one reason or another.

For example maybe you're playing a Bf4 QGD or a Meran semi slav (leaving the bishop on c1), but then in one or the other you are playing Bg5. So the book tells you to wait.

Objectively Bg5 is fine... and a common move (in both openings) at the GM level.

The best and most popular move in this position is 4. cxd5 followed by 5. Bg5

The opening explorer shows it wrong because 4. Nf3 leads to a very common position that is usually reached via the move order d4 Nf6 c4 e6 Nf3 d5 Nc3. But in your position it's more precise to delay the development of the knight because in some variations the knight will go to e2.

4. Bg5 is also okay, it's the 2nd most common move in the position.

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Sure, but being on the kingside means it's not on the queenside

So it's a tradeoff. In various openings sometimes black is playing moves like Qa5 Bb4 and Ne4 which all pressure c3 and the a5-e1 diagonal... this is generally not a good option if white can play Bd2. In other words Bg5 + e3 weakens white's queenside dark squares.

Of course "weakens" is relative here... I would hate for a new player to read this and think Bg5 is bad. Every move strengthens squares at the cost of weakening others. It's not a liability unless your opponent's pieces are positioned to take advantage of it.

Yeah, and for that reason many black players actually start with 3...Be7 (or recapture on d5 with the knight) even though the opening explorer probably shows 3...Nf6 as "best"

cxd5, Nf3, and Bg5 are all great moves. You will most likely develop a strong preference for one based on your opening repertoire.

Nf3 is the most flexible since it can lead to the standard Bg5 lines, Bf4 lines, Ragozin/Vienna, Semislav, exchange (minority attack plans), etc

Cxd5 is the usual route to the exchange lines since many of the standard plans for white require the white knight to go to e2 instead of f3

Bg5 is favored by those who play the traditional lines and don't want to allow the Semi Slav or other lines that Nf3 would allow.

I have always like Cxd5 as white, which is why I play 3...Be7 as black.  Even there, Nf6 and Be7 are considered equally good.

Yes that's right. A lot of d4 players like 3...Nf6 4c4xd5 but there is also 4...Nxd5 with semi-tarrasch positions. So there is no objective advantage with either of the three moves. Just what you prefer.

It would be better to go for a nimzo-indian right off the bat, but Bb4 still proves to be a very daunting game for White.

The Nimzo is just as good as the QGD, but less practical.  It takes more positional acumen to play well and a lot of players choose to avoid it entirely.  The QGD is difficult to sidestep if white plays 1.d4, c4, or even Nf3.