If i goto the store, Trine 4 says wishlisted and is available for me to buy, as though i don't own it, yet it is in my library, and i did buy it.

All the other games i own show "in library" on the storepage except for this, it used too until a few weeks ago atleast.

Same problem here.

By any chance, did you pre-order the game? If so, then that would account for it. When the game was released, the original store page was taken down and a new store page was put up for the post-release version of the game. The difference is that the pre-order version of the game includes the Toby's Dream DLC bonus level, which is not available to those who purchased the game after release.

A similar thing happened to me with another game. I purchased "Neverwinter Nights: Enhanced Edition Digital Deluxe Edition" last year before the "Tyrants of the Moonsea" DLC was released. Prior to that DLC being released, the store page for "Neverwinter Nights: Enhanced Edition" (the base game, which is part of the Digital Deluxe Edition package/bundle) showed that I owned it; since that DLC was released, the store page for NWN:EE shows that I do not own it and can buy it. When I asked GOG support why the base game no longer shows as being owned on the store page, the response I received was "it is not shown as owned due to a limitation of our system".

EDIT: It turns out that on 2019-12-10, Frozenbyte made the Toby's Dream DLC available to all owners of the game, so that would be why the store page changed -- it changed from the base game only to a package which includes the base game and the Toby's Dream DLC.

This happens if you bought the Ultimate Collection. All other games get marked as owned but not 4. No idea why.

Just that happened to me, I wrote to support, to which I was told that they are aware of the problem, and are investigating, however they could not promise me when the problem will be fixed.

Problem still not resolved after 2 years....Should we bomb them with bad reviews?