You asked the following:

Can you play the entire game in 4 player coop? I keep hearing the coop is bad, but want more specifics as to why

My answer:

The reason i dislike the games co-op is because it has tremendous desync when the host has more than 30 fps i play with a 7900xt and get consistent frames of around 200+ @1440p but my friends tell me they can't even open the doors at that point i tried capping my framerate to 60 fps which makes it playable they said but now i'm having a very bad time because it's at 60 fps.

And about the co-op cap it's been downgraded from 4 players in Dead Island 1 to 3 players in Dead Island 2.

Hope this clears it up.

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