A B747-400 Captain reported encountering what he felt was likely a wake vortex from a preceding B747 on approach; but also could be an autopilot fault.

Narrative

Aircraft upset while on autopilot. We were turning right base. The autopilot was engaged. While in a 25 degree bank; the aircraft suddenly and somewhat rapidly increased bank. I grabbed the wheel; disconnected the autopilot; pushed in left rudder and turned the airplane back toward wings level. I believe the bank angle may have exceeded 45 degrees and because of the quick and steady increase in bank; I believe the aircraft would have continued over on its back if we hadn't responded quickly enough. My assumption at the time was that it was the result of flying into the wake of the aircraft in front of us; [which was] also a 747. However; [later]; I wondered if it was from another cause. There was no turbulence at all coincident with the upset. It was very smooth. Second; we were very high and I believe [the other aircraft] likely was lower than us; but I'm not sure. (We had to use 30 flaps and gear down to get down because of the early base turn). I called Maintenance from my cell phone and explained what happened. I noticed from the [maintenance history] later in the day that they found the autopilot checked normal. So I'm at a loss to explain why this happened.