ACC unlocks additional cores. It's no longer a supported CPU for the temp program and the core temps readings don't work. Don't worry about it, use the board temp sensor.
As for why it boots, probably using a different core or some random thing. It's HIGHLY unlikely that it's stable at that speed, especially if it's not at 4.2. You need to run prime 95 for several hours
4.0 unlocked to a quad would be very good for that CPU. My 555 can't get past 3.7 when unlocked even though the temps are fine, no matter how much voltage I give it.