It's not the proximity sensor.
Have you tried holding home and power button for 15 seconds and try turning on the iPad on again?
If you see apple logo again then nothing to worry about, just tell the customer if it happens again just hold home + power button for 10 seconds. Explain to them that the software crashed.
I would say loose LCD display cable on the iPad 4 but highly unlikely as it uses a black flex cable that is taped on.