I have K.!
I think the thing that helped my preschooler the most was to talk about fears. We are religious, so we talked about giving our fears to God and that when we pray to Him and ask for His help He will comfort and help us. We also have a fear box, is a tangible object and makes the prayer a little more concrete for a 4 year old. I write the fear down (and then we say our pray here, but if you are not religious...) then put the little paper in the fear box.
It also helped to meet with her teacher and make some goals, about participating and trying her best in everything. She is afraid of trying anything new, cause she has never done it before :) (love that one)
Having her know that her teacher knew about her fears and goals, totally helper her realize that she can trust and feel safe with her, and not just me.
I also have a saying "Sometimes we all have to do things we don't want to do" I talked with her about why school is important and why it is a problem if she doesn't want to go or doesn't go. I also shared with her some of the things that I do even though I don't want to, and what would happen if I didn't do them... house cleaning, meals etc. This helped her realize she is not the only one that has these feelings, but that there is a way to concur them.
I also like to say "you don't really have a choice to go or not to go, your choice is to have a happy attitude and have fun or be sad and miserable, what are you choosing?"
These conversations were of course at different times and many times. But she finally decided that it wasn't such a bad place to be.
Your little guy is 4 and it is such a great time because you can actually reason with him. Try a goal chart that if he cooperates for a week he can earn a treat, then after two weeks dollar toy then three weeks a date with mom or dad. keep building on that until it has been a month and you have your happy child back.
Good luck, message me if you have questions.
E.