Well, you dug yourself into a hole by converting the footage with Compressor. Now there's no way to track the source, not in FCP. If you have 4K footage, and you want to track it and finish with it, you need to convert it in certain ways.
If you intended to finish in FCP, then you needed to convert thru FCP...via log and transfer. BUT, if that wouldn't work, and it might not as FCP is old as the hills and not up with current camera tech, then you would have to look at finishing outside of FCP. If you wanted to finish this with Color, you should have used COLOR to convert the footage. HOW? I don't know, I haven't used Color in that way, but I know it's possible. If you wanted to use Resolve, then you should have used Resolve to convert the footage. This is how they track the offline media to the full resolution masters.
If you convert with something like Compressor, MPEG STREAMCLIP, EditReady, BulletProof...you lose all tracking to the masters.
So your only solution now is to convert ALL the footage you used in the cut to 4K ProRes files. The full clips...and then see if you can relink (might not be possible if you don't have matching reel numbers, and you don't because you used Compressor)...and if not...then manually over-cut.
Sorry you got yourself into this corner. Offline/online workflows require planning (what will it be finished on) and certain workflows so that you can track the footage and relink it when you go to finish.
Good luck