This does not work, at least for me.
When I log into Mog Station in a Squenix account with an active PC license and click the "Apply PS4 License" I get this:
Add PlayStation®4 License
To register the expansion for FINAL FANTASY XIV on the PlayStation®3 or PlayStation®4, you must purchase the digital download, or apply a registration code through the Sony Entertainment Network.
Upon completing registration and installation, start the game and follow the on-screen instructions to log in and add your license to your Square Enix account.
*The product code cannot be registered via the Mog Station.