The imagination of a child can be both amazing and frightening. I have tried with my own children to try to channel those (usually irrational) fears into their creativity. Trying to get them to elaborate on their dreams, or draw a picture of what they saw.
Did he by chance watch X-men, in the 3rd installment there was a "porcupine" guy with spikes sticking out all over him.
I would hope your four year old isn't watching Hellraiser, so I don't think that's where he's getting the image from.