For D4 players, as white, when is C4 NOT your second move?

Beginner here. I've been playing 1. D4 2. C4 almost exclusively as white and am wondering what other moves you guys like as your second move. For example, I know the London is popular and a D4 opening but in what situation would I want to play that as opposed to just going C4? Is it just preference?

Depends on what opening you wanna go for, you can play the colle with 2. Nf3 the london, with 2. Bf4 you can play the veresov with 2. Nc3 you can play 2. e4 and play a french by transposition, all of it just comes down to what you want to play. The colle and london are solid the veresov is attacking and the french is sound

@Sluump I like 1. d4 2. Nf3 3. g3 4. Bg2 5. O-O 6. c4 (or 6. Bf4 7. Nbd2 or Re1 or Bxd6 if black plays Bd6), where black can play 6... dxc4, leading to a more open game, or just hold on to the center (if ...e6 or ...c6 was already played).

It leads to quite interesting positions and is perfectly sound...

@Sluump I'd play 1.d4 d6 2.Nf3 as white because of 1.d4 d6 2.c4 e5 when 3.dxe5?! isn't great.

@Sluump 1. d4 d5 2. Nf3 Nf6 3. g3 g6 4. Bg2 Bg7 5. O-O O-O 6. c4 c6 7. Nc3 dxc4 8. e4 is a pretty interesting line. Generally speaking, 1. d4 d5 2. Nf3 usually transposes to the QGA or QGD, Slav Mainlines, Catalan Opening, or a hybrid of sorts...

@SavageAntarctican of course it's fine for white, but it's one way for black to exploit this, which IIRC is covered in the book "Beating 1.d4 Sidelines" by Boris Avrukh. The other way to exploit this line is to play a setup with Bf5 and c6, arguing that the bishop is badly placed on g2 in a Slav setup.