user2984: [FEN ""] 

1.d4 d5 2.c4 c6 3.Nc3 Nf6 4.Nf3

Here we reach slav defense main line and theory goes with 4...e6
As a way to avoid theory as black I used to play in blitz 4...g6, goes with bishop fianchetto and castle.
As white I'm a d4 player and although not many people deviate 4...e6 with 4...g6 this could happen.
Searching in professional games I didn't see many games with 4...g6.
My question is: How white proceed after 4...g6? (there must be a reason why professionals rare play this move)

user-1: I play both sides of the slav-defense and have not come across 4...g6 as the dark square bishop usually belongs on b4 (in the mainline Dutch), d6 (in the exchange) or maybe e7.
Having said that, this is how I would play
[FEN ""]
[StartPly "8"]

1. d4 d5 2. c4 c6 3. Nc3 Nf6 4. Nf3 g6 5. cxd5 cxd5 (5...Nxd5 {This transposes to the Grunfeld} 6. e4 Nxc3 7. bxc3) 6. Bf4 Bg7 {Aiming to play Qc1, Bh6 and trade dark square bishops} (6...Nh5? 7. Bxb8 Rxb8 8. Qa4+ {This wins a pawn}) 7. Qc1 (7. Qd2 Ne4 8. Nxe4 dxe4 9. Ng1 Qxd4)

N.B. This analysis has not been checked by an engine

user14202: The main reason 4...g6 is not played is because White can put his dark squared bishop out side the pawn chain (Bf4) followed by e3. Blacks fianchettoed bishop will not have good influence as d4 is firmly held.  Whites dark square bishop however will influence the queenside where White will be seeking an advantage.
You will see more games with g6 when White has played an early e3 blocking the bishop.  Smyslov played some nice games on the Black side of the Schlecter -- see his 125 Selected Games book.

user10295: According to this database: 
The next move for white will be:

5.e3 with 40% of win, 46% of draw and 14% of lose. (3538 games were played like that)
5.cxd5 with 48.5% of win, 41.5% of draw. (344 games were played like that)

Example for a game which start like this:
[Event "1st Rottal Open 2017"]
[Site "Pfarrkirchen GER"]
[Round "6.1"]
[Date "2017.4.17"]
[White "Haba, Petr"]
[Black "Riediger, Martin"]
[Result "1/2-1/2"]

[FEN ""]    
1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 g6 5.cxd5 cxd5 6.Bf4 a6 7.h3 Bg7 8.e3 Nc6 9.Be2 O-O 10.O-O Bf5 11.Rc1 Ne4 12.Na4 Bd7 13.Qb3 b5 14.Nc5 Nxc5 15.Rxc5 e6 16.Bd6 Re8 17.Ne5 Nxe5 18.dxe5 f6 19.Bc7 Qe7 20.Bd6 Qd8 21.Bc7 Qe7 22.Bd6 1/2-1/2


2984: <pre><code>[FEN ""] 

1.d4 d5 2.c4 c6 3.Nc3 Nf6 4.Nf3
</code></pre>

<p>Here we reach slav defense main line and theory goes with 4...e6</p>

<p>As a way to avoid theory as black I used to play in blitz 4...g6, goes with bishop fianchetto and castle.</p>

<p>As white I'm a d4 player and although not many people deviate 4...e6 with 4...g6 this could happen.</p>

<p>Searching in professional games I didn't see many games with 4...g6.</p>

<p>My question is: How white proceed after 4...g6? (there must be a reason why professionals rare play this move)</p>
 -1: <p>I play both sides of the <a href="/questions/tagged/slav-defense" class="post-tag" title="show questions tagged &#39;slav-defense&#39;" rel="tag">slav-defense</a> and have not come across 4...g6 as the dark square bishop usually belongs on b4 (in the mainline Dutch), d6 (in the exchange) or maybe e7.</p>

<p>Having said that, this is how I would play</p>

<pre><code>[FEN ""]
[StartPly "8"]

1. d4 d5 2. c4 c6 3. Nc3 Nf6 4. Nf3 g6 5. cxd5 cxd5 (5...Nxd5 {This transposes to the Grunfeld} 6. e4 Nxc3 7. bxc3) 6. Bf4 Bg7 {Aiming to play Qc1, Bh6 and trade dark square bishops} (6...Nh5? 7. Bxb8 Rxb8 8. Qa4+ {This wins a pawn}) 7. Qc1 (7. Qd2 Ne4 8. Nxe4 dxe4 9. Ng1 Qxd4)
</code></pre>

<p>N.B. This analysis has not been checked by an engine</p>
 14202: <p>The main reason 4...g6 is not played is because White can put his dark squared bishop out side the pawn chain (Bf4) followed by e3. Blacks fianchettoed bishop will not have good influence as d4 is firmly held.  Whites dark square bishop however will influence the queenside where White will be seeking an advantage.
You will see more games with g6 when White has played an early e3 blocking the bishop.  Smyslov played some nice games on the Black side of the Schlecter -- see his 125 Selected Games book.</p>
 10295: <p>According to <a href="https://chesstempo.com/game-database.html" rel="nofollow noreferrer">this database</a>: </p>

<p>The next move for white will be:</p>

<ul>
<li><p><code>5.e3</code> with 40% of win, 46% of draw and 14% of lose. (3538 games were played like that)</p></li>
<li><p><code>5.cxd5</code> with 48.5% of win, 41.5% of draw. (344 games were played like that)</p></li>
</ul>

<p>Example for a game which start like this:</p>

<pre><code>[Event "1st Rottal Open 2017"]
[Site "Pfarrkirchen GER"]
[Round "6.1"]
[Date "2017.4.17"]
[White "Haba, Petr"]
[Black "Riediger, Martin"]
[Result "1/2-1/2"]

[FEN ""]    
1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 g6 5.cxd5 cxd5 6.Bf4 a6 7.h3 Bg7 8.e3 Nc6 9.Be2 O-O 10.O-O Bf5 11.Rc1 Ne4 12.Na4 Bd7 13.Qb3 b5 14.Nc5 Nxc5 15.Rxc5 e6 16.Bd6 Re8 17.Ne5 Nxe5 18.dxe5 f6 19.Bc7 Qe7 20.Bd6 Qd8 21.Bc7 Qe7 22.Bd6 1/2-1/2
</code></pre>
