If I understand correctly (and I’m not 100% sure I do), localhost in a Docker container lives in it’s own little network which is not the host’s network.
The container is its own localhost, which has its own ports (which is why you have to map an internal localhost port to a host PC localhost port for every container you wish to access). This means that Prowlarr in your case, has no idea what localhost:4666 should be since in Prowlarr’s localhost universe there exists nothing on that port. To access what the host knows of ports (instead of the container), you have to write the host’s address from inside the Prowlarr container.
I hope that wasn’t impossible to follow 😅
Now that I think about it (haven’t tried myself though) you could possibly add the mapping of port 4666:4666 to the Prowlarr Docker compose setup and then use localhost:4666 to access qBittorrent from inside Prowlarr.