Ask him why he does not want to go any more.
My daughter also 4 and in preschool went through a phase like this.
If he says he does not want to go ask why. Let him know he can tell you anything.
My daughter had a few reasons:
-- There was a boy being mean to her... in this case we talked about how to handle it, guess what they are friends now and he is no longer mean to her.
-- She is bored at preschool. Everything that they are going through she already knows, serisouly she is doing math problems and is reading at a 2nd grade level already. So we talked about listening to the teacher and that going to school is good even if you know what is being taught that day already. She is an only child so she also knows that this is a time for her to be with kids her age, but she has to only play when the teacher says it is ok. My daughter is going into kindergarten next year, she turns 5 in Aug.
-- She misses me. I ask will she miss her friends if she stayed at home with me all the time, the answer is yes. So I reminder her that I am thinking of her every minute she is in preschool, give her a big hug and kiss and tell her what time I will be there to pick her up, usually around 11:30am. This lets her know that even though I am going I will be back... this is an age where kids start to worry about death, being left alone and so on so you may need to just give reminders that you will be back and when.
I think partly he is use to things always changing since he has been to 3 different ones in a year. So this is new to him and might be getting bored with it. I would stick it out till the end of the school year and see how he feels about it then.
If he can not come up with answer to why he does not want to go then maybe ask the teacher or the helper how he is in preschool. They might have some insight for you.