Too many changes at once...mom "kicking" him out of the only bed he has known and sending him to preschool. His world has been totally turned upside down. He is reacting the only way a 4 yo knows how. Not saying his actions are right, just that is the only way he knows how to show his upset.
First, start gradually working him out of your bed. You made the decision to have him in your bed, why punish him because you decide you don't want him there any more?
My 5 yo has only been in his own room and bed now for 10 months; successfully all night long, going to sleep in his room about 6 months, so I know how difficult that transition can be.
First, go shopping and let him pick out a new bedding set (sheets/blanket, pillowcase, etc) for his big boy bed. Make a BIG deal about what a big boy he is now and getting the new stuff for his big boy bed. If he is still in a toddler bed, maybe the mattress isn't as comfortable for him as the regular bed mattress. This was part of our problem. Once we got him a twin bed with a regular mattress, he hasn't come back to our room except for when he is really sick.
Start with letting him go to sleep in your room then carrying him to his bed once he is fully out. Some nights he may come right back to bed with you, some nights he may come back at 1 a.m., and some nights he won't come back at all. However, when he does come back, let him and try again the next night. Each night do the same until he spends a full month in his room without coming back every night.
Once he sleeps a full month in his room without waking, then start putting him to bed in his room. Sit with him, no talking (make that rule clear that if he isn't quite and still you will leave and he will have to be by himself, give a few warnings/reminders as needed till he understands and gets used to the new routine). Go a couple weeks like that and then start slowly reducing the time where you are leaving just before he falls asleep, then to only staying a few minutes, then to only tucking him in/reading a book and saying good night.
Get some sleep music for him also. This has helped our son not to feel so alone. We also let him take 5-10 minutes before bedtime to pick out sleep toys. He isn't allowed to play with them and doesn't (they will be taken away if he plays and he knows this), however, it allows him to have something in bed with him. Most nights, there is quite a pile of toys LOL (which we move around to allow room for him to move once he is asleep). He is now to the point that after we tuck him in he drifts on off by himself, some nights he is awake longer than others, but he is quiet and resting. We also leave the hall light on so it isn't dark in his room.
Good luck and just take it slow. He has spent 4 years in the bed with you. The transition isn't going to be easy for him and it isn't going to be quick.