I've never used PF Portchecker, but using canyouseeme.org will do the same thing from outside your network, which is what needs to be done.
There's no reason to fwd port 80 if Subsonic is using 4040.
the process is always the Same:
Install Subsonic.
Check to see that it works on the local host.
If you have acces on the local host's browser, then test on another device on your local network. if it won't connect then you have to edit the firewall on the local host.
If it wordk inside the network on a client device, then test from outside the network. If it doesn't work then your port forwarding is NOT set up correctly on the router.
If you CANNOT get a successful port test from
http://canyouseeme.org you will NOT be able to connect from outside your network. It's just not possible.
90% of the connection issues addressed are the result of improper port forwarding. Look at any rules currently working on the router for examples of proper set up. Read the manual. If it's done properly you WILL get a successful test from canyouseeme. Check other active rules if your not convinced.
Router entries can be confusing. If you can't get it working, post your routers make and model and someone might be able to assist you by finding the manual online and help identify the error.