I'd suggest a checkup at the doctor. He could have one of those silent lurking illnesses like an ear infection that's making him feel out of sorts (and if you've ever had a high temp and hallucinated at night that's pretty scary too).

I would also be looking out for something that has rocked his sense of personal safety. It's all very fearful behaviour. Is he dealing with any big life transitions (starting kindy, family member has left, moved house, etc)? All of these things can rock their sense of security/stability too. Try giving him some comfort rituals, or talk about how safe and sound he is in bed, etc. Try a night light if he doesn't have one or keep the door open for him.

Perhaps he has been worrying about something he has heard or seen that has not made sense to him. I remember being hysterical when I was 4 because there were some moths in the garden and someone had told me they eat holes in your jumpers and I was wearing a jumper and thought they'd jump on me and start munching LOL - that's how the 4 year-old brain works. In the meantime, try not to worry too much yourself, it could well be a passing phase.