Why ...c4 Here, and What is the Plan?

My .02 for what its worth.

Limits the scope of the LSB.

The d-pawn is permanently weak.

Controls b3-d3.

Gains space as black has a 4-3 queenside pawn majority.

Forgot one more:  Increase the activity of the DSB.

And yes "in general" you dont want to release the pawn tension unless you gain something from it.  Here i think black gains enough to release the pawn tension.

Thanks IMKeto

Which means squat if you dont understand why.

I agree, it gets blacks q side pawn majority moving and gains space. It makes whites bishop worth less than it was a move before (just count the squares it influences after c4 and it moves). 

     With a queenside pawn majority and white's kingside majority unable to force a passed pawn (all three files on which the pawns stand occupied by black pawns) any eventual K+P endings should be winning for black. This gives black a middlegame advantage because exchanging pieces helps black reach that won endgame.

You can also look at it from white's side... after c4 how does white come into contact with d5? Meanwhile black can come into contact with d4.

The high eval can be misleading since you have to play super accurately to make it true, but I think many players should be able to tell, when looking at white's pieces after c4, that white is passive and awkward. So c4 is an attractive move. Let's call it 0.5 to 1 in black's favor (and over 2 if you're a titled player in a 2 hour game )

limits white's knights. If white plays Ne5 simply Nxd4. If white plays Nf1, it looks weird and maybe Bd6 is good enough for a punish, targeting the weak f4 pawn. And of course Bg4. 

Thanks for all the replies. It's interesting that strong players are giving different reasons why ...c4 is good, although these reasons are not contradicting each other.

Another consideration is that if Black doesn't play ...c4 then White can play d x c5 and after Bxc5 Black has an isolated pawn on a semi-open file. I believe these are called "isolani" positions which are not necessarily bad but it looks like a game of baby-sitting rather than a strong passed pawn to me.

Another question - if White's f4 pawn were instead on c3, would ...c4 still be a good move for Black?

Yeah, moving the f pawn to c3 fixes two of white's problems at once:  f4 and d4 were targets.

In that situation it's a bit reversed from what I described above. White can come into contact with d5, but black has no targets. Sure black could finish development and then push b5-b4, but this would take, I don't know, a minimum of 5 moves, maybe more like 10... so it's way too slow. c4 also gives white the b3 pawn break.

So no, c4 doesn't look very good in that case.

You'll notice the common theme in my answers is looking at whether or not minor pieces can come into contact with weak enemy pawns.

It's also worth considering WHEN ... c4 is bad.

Sometimes it's bad because it overextends the Queen's side Pawns and allows a counter attack with b3 and a4.

More typically, it's bad because it releases the central tension; when White can arrange to later play e4 striking at the d5 Pawn, or when he can make use of the stabilization of the center to launch a King's side attack.

None of those conditions applies here.

Yeah, that's a much more typical conversation.

In this position it's more like black has pressure on squares like d4, f4, and even b2, meanwhile the knight on d2 is being embarrassed by the d5 pawn... in fact if I were going to change something for white (other than f4 to c3 which is obviously good) I'd put the d2 knight on c3.

Thanks. In the positions in "Logical Chess MbM" White usually had a pawn triangle c3, d4 and e3 against Blacks on C5, d5 and e6 - with all the others on their home squares. Then Black would play ...c4?! and later White would play e4 (then e5 and make make a greek-gift sacrifice on h7). The player with the White pieces was then treated as a hero and the crowd would buy him drinks all evening, whereas the player with the Black pieces was the villain for playing ...c4 and the crowd would poke him with sticks.

But I may be mis-remembering, it's probably 35 years since I read that book.

Don't think any big mystery here, after ...c4 black is eventually winning material because d4 and f4 weak, especially d4.

There are cases when the pawn structure with f4, after giving up bishop for knight is good for white, or at least double edged. See old games of Rubinstein and Alekhine. It's not the case here.

That would be my main reason for playing ...c4 in that position.

Allowing white to exchange on c5 would simply give white too much. They'd have the isolated d5 pawn to maneuver against, and the d4 square to blockade.

Playing ...c4 here has no real drawbacks for black. It wins a tempo on white's bishop, further cramps white's position, solidifies black's central pawn, and doesn't impede on any of black's bishops.

The only possible negative is that it leaves black's d5 pawn backward. But with white's d4 pawn in front of it, the backward d5-pawn is safe from being pressured by a rook, and safe from being blockaded by a piece.

True but if a weakness cannot be attacked then its not a weakness.

So in general a move like ...c4 is not the right idea in most positions. This is because you give up the tension in the center, basically handing it over to your opponent. This position is different. 

The position can in part be answered by looking at the weaknesses in white's position. The pawns on d4 and f4 are weaknesses. Pawn d4 can't be defended by other pawns anymore and can be attacked. Since you don't want to trade off a weakness of your opponent for a perfectly normal pawn of your own, ...c4 is a logical move. 

I think the reason for liking black so much is deeper still though. Black just has a ton of advantages. Pawn f4 can only be defended by g3, but that leaves the white squares very weak. Black has no weaknesses, white has d4 (as already noted). Black has more space. Black has the bishop pair in a semi-open position. White has a rather useless doubled pawn, while black has the queenside majority. The white pieces just aren't active. Where are you going to put Nd2? What is Bc2 attacking? And I think black has an initiative, because he'll quickly develop and then start putting real pressure on white's weak pawns, while white's pieces lack a certain harmony, making it hard for them to defend the weaknesses. Basically: black has everything. 

Nice dissection of the position.

Thanks to all of you who took the time to reply. They made a lot of sense.