Success! I had connected a 2nd SSD with an old installation of Yosemite that I was going to reformat for Windows. I unplugged it and after booting up, have two fully working 4k screens. The NVIDIA Driver Manager, however, doesn't recognise the GTX 960 in any of the slots and G-Sync is disabled. It also says that ECC is not supported. Sound was only working from USB and it didn't wake from sleep. However, I've been searching the site for the answers and modifying the config.plist and info.plist accordingly. About My Mac is now showing the full 4Gb graphics. Both displays are recognised.
Thanks for all your help!