Yeah, but not really a spirit guide. Apparently Time Lords really
skilled in regeneration can project their own future self as a separate being, molding and tweaking them to their own liking, then merging together with the projection come the critical moment. That may have been what Four was seeing in the opening moments of Romana's regeneration - the projections coming out for his approval while Romana was preparing to change back in her room.
Four (who wasn't skilled at all) subconsciously created such a projection - an ghostly, unformed "Watcher" - on his last adventure. Or, as some speculated, Five projected the Watcher backward in time to help Four against the Master (and also gathering his own future Companions together) while reminding him "Hey, time's up, you've had that body long enough, it's my turn now."