Blizz didn't really intend to have 4 socket slots, so the UI doesn't really support it, same thing happens if a helm with 3 socket slots upgrade with an additional socket.
I havent tried myself, but is the UI the only issue? if one were able to write a script or weakaura/UI extension addon to socket the 4th slot, do you think it would work then?
That's gonna be hard to find a solution without having access to a char that has the feature, to test random stuff ^^
I'm gonna try to find things to tests
the socket helper addon will highlight unused sockets and can place in sockets with a single click without opening up socket window. Have not tested on a 4th socket
my assumptions would be that if the code for this addon didnt work for a 4 socket item out of the box, and if we could get access to the lua code behind it, we might be able to adjust the code for finding a 4th socket/ placing the gem there
I'm not on the discord either, but tried through PMs without any answer yet
Currently the plan is, while having the socketing window open:
1) paste /run print(GetNumSockets()) to check if that 4th slot exists for real
2) if the answer was "4" then do a left click on the gem you want, then paste /run ClickSocketButton(4) to see if it adds the gem and changes the tooltip