What do you like to play against the Catalan as Black?

4...dxc4 5.Bg2 c5 is perfectly good, and Black can choose in the main line if he wants a complicated position, or "keep it simple".

@1
Accept the pawn and let white prove his compensation.
https://www.chessgames.com/perl/chessgame?gid=2122755 

I like the Catalan as white but have trouble finding anyone who plays into it.

4..Nc6, and 5..dxc4

Christof Sielecki has a whole chapter on it for Nimzo-Indian players in his "Lifetime Repertoire: Nimzo-Ragozin" course on Chessable.

Damian Lemos has another approach (based on the Bogo-Indian) in his Nimzo/Bogo Indian video series.

Depending on what you play as Black, those would be good places to start.

You are forcing the Catalan too much when you play. In Christof Sielecki's Keep it Simple 1.d4, He recommends against almost anything that Black plays to play d4, Nf3, g3, Bg2, 0-0. Later on, then you play c4 which prevents a lot of early sidelines and it goes straight into your territory

I like to play QGD and sometimes when I see that a8-h1 diagonal will be open I transpose to Catalan... Slav defense is something that stops me to transpose it to Catalan...

I am 1500 rated player so not so good with theories and similar to your rating so that maybe help you... So, with Slav defense you will probably avoid it or play against it effectively on this level.

The Slav does not prevent the Catalan, nor it's an easy equalizer. Actually the Catalan is quite popular as an anti-Slav weapon.

https://www.modern-chess.com/expert-repertoire-against-the-slav-defence-part-1-1-5h-running-time-270

Of course you are right, what I tried to give here is personal opinion of mine, on a pretty low level... He is around 1400, I am around 1500 and I know I have troubles with catalan against slav... So, when I play catalan I hate to face slav defence... For sure I am no good in openings neither in other stages of the game with this rating and I just know some basic principles of catalan, like couple of moves...

In club player play it seems one of those openings that shouldn't worry black much. Opponents just won't have the technique necessary to exploit very subtle positional ideas. The main problem for black might be the game is little dull. The above is an exchange sac idea that was mention by IM Pfren some years back, which unbalances play.  

There is also something to be said about the stereotypical "triangle" approach against the Catalan. The line was not popular at all directly, although Black sometimes opted for this setup inserting Bb4+ before Bd6, so that the Bd2 would prevent the "advantageous" Nfd2/Nc3 setup from white.

However, it seems that this finesse isn't really necessary, and Black can equalize:

11...Bb4! is a fine novelty, which is not mentioned by Avrukh and Bologan, and gives Black plenty of compensation for a pawn, or even an exchange, mainly due to the strength of the passed d4 pawn. The guy playing Black in this game is the current World U-12 champion, and also the highest U-12 ELO (currently 2376).

IM omg sign my London opening

I usually go for an early c5 and Rb8 as soon as possible. It's all about stopping those queenside pawns.