If the engine (for example the BMW 801 which required C3) was run at a normal power level for C3 and B4 was in the tanks by chance; there would be detonation involved.
SuperchargersOnline.com :: Detonation, Knock, and Pre-Ignition 101
A number of things could happen. In very little time, the top of the pistons would burn through most likely.
Now if it was detuned to be able to run on B4 which would reduced power , then there would be no problems. Expect the power would be reduced significantly though.
Running a higher rated fuel than an engine can use or set up to use would be no problem. It wouldn't make more power just from the fuel though. The engine could produce more power, but would have to be set-up to properly to make it. Higher boost pressures, advance in the timing, proper fuel mixture and a change to cooler plugs probably. Of course the said engine must be strong enough to handle the higher power.
As for the Japanese aircraft and producing more power on US 100/150...well it's possible if this fuel was of higher octane than the Japanese variety. If someone could find the fuel tests of the Japanese fuel(s), then we may take a stab at the answer.