I never played these games co-op but had a blast by my onesy self. I honestly didn't hate Trine 3 as much as the mass did. Yes, as a whole, it was the weakest of the 3, but it wasn't by any means a bad game. I think they even released some additional content post release and overall, it wasn't as quick to get through as many would have made you believe. It was reasonable length.
Glad to hear however that they learned from 3 and will not be going that route on 4. Taking what made 1 and 2 great and only building on top of that means Trine 4 is gonna be awesum. Can't wait.