[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)