D4 D5 C4 NF6 Continuation I'm a quiet and positional player rated in the 1400's and play exclusively online correspondence chess. I always play D4 with white as first move and usually C4 after that.

One opening I come into quite often is D4 D5 C4 NF6

After this what is the most solid play for white to get a strategic advantage for long term correspondence play?
This opening 1.d4 d5 2.d4 Nf6 is known as the Marshall Defense of the Queen's Gambit Declined.  White can get an advantage with 3.cxd5 resulting in an advantage, such as 3...Nxd5 4.Nf3 Bf5 5.Nbd2 Nf6 6.Qb3 Qc8 7.g3. 

However a lot of those lines involve wide open tactical play, and you are looking for a quiet positional alternative. Therefore simply 3.Nc3 when Black can choose between several different openings.


The main line Queen's Gambit Declined with 3...e6 4.Bg5 Be7 5.Nf3.
The Slav with 3...c6 can get tactical, so maybe play 4.e3 Bf5 5.Nf3 e6 when 6.Nh4 is popular, to get the advantage of two Bishops. 
The Queen's Gambit Accepted 3...dxc4 4.e3 or 4.e4 and you should easily regain the pawn.
The Grünfeld Defence 3...g6 can also get quite tactical. 4.Bf4 Bg7 5.e3 O-O 6.Rc1 is one way to steer the game away in a quieter direction.