Is the GOG-Version updated to 4-player coop or is it missing that official update?

Well, to answer my own question: YES :-)

They updated the main description, I think.
Now it starts with: "LOVERS IN A DANGEROUS SPACETIME is a frantic 1- or 4-player couch co-op action space shooter. "
... says it all I guess.

Nothing in the changelogs, though.