I am having a some friends come over for a gaming party to hang out and kill stuff.

I found some good games on Steam like rocket league etc. But i do like buying GoG version where i can.

Any recommendations on what 4 player local co-op games are available on GoG that are really fun?

Thanks

I f you are only interested in 4 player local co-op games, then I can recommend only one from GOG, the Flatout series, it can go up to 8 players. I still play it with friends from time to time.
Gameplay

Megabyte Punch also has a local 4 player co-op mode.

Judging from the screenshots, Screencheat seems to have a splitscreen mode for even more than 4 players.

Towerfall Ascension, get it's it's amazing.

Hi guys

+1 for towerfall

But one of the best local coop here are :

Risk of Rain
Crimsonland

Enjoy

If split screen isn't a hard requirement, look into Hammerwatch. It supports four player co-op, though you're all on the same screen. However, it's on the "second class citizen" GOGMix because it hasn't been updated. Beware.

Rayman Legends can be 4 player.

Would be nice if GoG can get their mitts on Rayman Origins.

Other way around. :P
We have Origins here but not Legends.
There's no real reason to play Origins though as most of the levels are unlockable in Legends.

I must be blind. My apologies. I've edited the original post.

Thanks for the correction. Son has both on xBox.

Mind you my answer is wrong for the OP since he's asking for split screen.

Four players split on one screen?

Are you playing on a Jumbo-Tron? :P

4K-screen, everyone can play in full HD :-P

Actually, that would be nice, I should get a 4K projector to replace my current 720p one.

A tad OT, and assuming you know what a Jumbo Tron is (I know Europe has them but they might have a different name), a few weeks ago I saw a Make-A-Wish story about a kid who got to meet his favorite player and they went out onto the field with two chairs and actually played Madden on the Jumbo Tron. XD Way Cool.