If @BandOfBrothers solution isn't available, there are two things you could try. The first is to simply long press the power key and see if that forces a powerdown, though I suspect the initial issue will return after restarting.
It's also possible that you may be able to reset the device via recovery mode. Press and hold volume down and then power until it starts (or restarts if you couldn't turn it right off). With luck, that'll produce a text screen and you can move through the options with the volume keys and use power to select. Simply choose the reset one and the phone should revert to its out-of-the-box state.
A 4-digit code doesn't sound like a network lock as, from memory, a PUK code is longer than 4 digits.
I had a Samsung tablet bought as an Amazon warehouse deal that stopped responding to the touchscreen after a system update and would only restart (long press on power key), but not switch off. I was able to use recovery mode to wipe it (as above) before returning it.
