Yes. Four's last companion. I think so.
I think Four "faked" his regeneration.
"It's the end-- but the moment has been prepared for…"
And then the Watcher merges with the Doctor.
Or maybe he took the Doctor's place and the Doctor went off somewhere else to not die.
Curator is suspiciously close to Watcher IMHO.
Do Four retires from doctoring for a while -- perhaps to watch over Gallifrey -- perhaps to just retire on Earth for a while.