Hi T.,
I know as a mom of two boys how frustrating it can be when we can see a bad habit in our children that we probably helped create. If we could only rewind the clock.
But, what is, is and now we have to working on correcting our mistakes. Thank goodness for GRACE!
There are a couple things to keep in mind. You are the parent and he is four. So ultimately you are in charge and should be able to create an atmosphere where you as the parent get the results you want. It is also important to remember that this is probably built around some fear for him of being alone and the need for comfort. This is not (most likely) about defiance. Unless, of course that is his typical behaviour which is a whole other discussion. So handling it in a nurturing way would be my first route.
I'd try and create something special in his room that makes him feel comforted. Like maybe a new (or used) bedspread with a stuffed animal to help keep him company. Or, get a special night light. Or, paint his room a special color that he likes. Or, develop some type of special reward system where in the morning if he makes it through the night you would fix him his favorite breakfast. Or, if he makes it 5 nights in a row he gets something bigger.
My guess is that if you can show him it is not scary, that he still can crawl in bed with you in the morning and that the comfort is still there, he will start feeling more secure about being alone.
I remember worrying about taking my son's pacifier away. I waited and waited because I didn't want to have to "deal" witht he screaming and crying. Well, it took one night. Yes, he cried for about an hour, but that was the last of it.
I hope this has helped in some way T.. Again, the key in parenting is to remember that YOU are the parent. He is the child.
Blessings!