If you teleport to a room that doesn't exist, you go to 4,4 instead. A hint for 15,8 can be found at 9,20.
So I assume that means there's a room somewhere that rather than following the usual rule for wormholes will land me in 4,4? Because I've searched, and every room I've found that might fit that criteria (now that I've uncovered the rest of the map) doesn't allow me a spare cat as far as I can tell. So I must be missing something that's obvious-in-hindsight.
Last thing I need to finally 100% this beast...