His statement said: 4-weeks+ → /avail

The contrapositive says to negate both and switch the sides so that would be the same as yours: avail → /4-weeks+

"Any newly arrived cat at Anjellicle Cats Rescue will not be available for adoption if there are other cats waiting for longer than four weeks. Nittens is available for adoption."

How it helps me is to read every arrow statement as "if/then". So, for this one, when I read the sentence, I tried to get it to fit into this "if/then" phrase. It already says "if there are other cats waiting for longer than four weeks" so that part fits in easily. if that is true what THEN happens "then any newly arrived cat will not be available for adoption". So when I write it out, I put it in that order of "if → then".

There are other cats waiting for longer than four weeks → any newly arrived cat will not be available for adoption

To shorten it : other cats 4+ → /adoption

The contrapositive of this is: adoption → /other cats 4+

For how you spelled it out, it would read: if a newly arrived cat is not available for adoption, then there are other cats waiting for longer than four weeks. But this is not necessarily true. If there is a newly arrived cat that has worms or needs some kind of vet care, it may not be available for adoption. We only know that if there are no other cats waiting, there is the POSSIBILITY of the new cat being up for adoption, but it isn't guaranteed.

Has this been made yet?