My suggestion is to hear his pleads for security. He is only four - let him stay in your bed/room.
Why do parents always think they know what is best for their kids ? Sometimes we need to hear our kids and stop dismissing their concerns. Your child is four not 44. You will one day regret forcing him to stay alone in his room petrified -- why not comfort your child. Don't even question if they want to stay wherever they want. By forcing your child because you think its the right thing to do, you are making the situation worse.
It's your fault that you were not paying attention to what your 4 yr. old was being exposed to. You shouldn't let the 12 yr. old unattended on the computer first of all - and zombies ? Why would you even allow a 12 year old to see that ? You petrify your little one and then expect to send them off to the dungeon alone. Not right.
You know the right thing to do, stop reading all your parenting books about letting the child suffer in silence. Hug your child, let them stay in your room until the day comes when they are secure enough to try staying in their own room again.
Don't undermine your child's fears by saying there is nothing to be afraid of. That is nuts. He obviously HAS fears, and justified fears - to dismiss them like you are is to make him doubt his own thoughts. You can tell him, 'yes I know that is scary, but I am right here with you, and will protect you....'
Be a loving gentle parent here and do the right thing.
****Mommyof2 -- I agree with you 110% !!! Adults always can have someone else in their room, yet the kids are sentenced to solitary confinement by many. Sad situation. My kids have an open door any time any day they want to my room. I am so glad I am here now to comfort their fears.... I don't get why people have kids and then don't want to deal with the little ones' issues....