His weight is on the 50th percentile and his height is just below the 90th percentile for his age which indicates he is normal for his age. He may appear thinner due to his height. Suggest noticing the times when he stops chewing his food, such as did he have similar days with friends or daycare, etc. Look for patterns and see if that will help you. Suggest giving him foods he likes when he starts to not chewing and do not make a fuss when it occurs. Hopefully he will stop the behavior if you do not really pay no attention to it. Hoped this helped you.
My son is four also, and he has sensory processing disorder. He will eat nothing that is creamy in consistancy, and has been this way since he was about 2 or 2 and 1/2 years old. Around a year ago he started spitting out chicken nuggets, spagetti, salad, corn, al things he normally ate and ate well. Because we knew he had some problems with textures and sensory issues we went to a food therapist, but nothing really changed anything. Gradually he started to eat those foods again without spitting it out, a couple of things he is still weird about not wanting to eat. By weird I mean it like he really looks like he wants to eat it but like I can't. I remember he was that way when he went through the spitting out phase. He absolutely loves salad and he would get so upset when he "had" to spit it out, he would even try again only to spit it out again, but he always seemed like he wished he could have just swallowed and enjoyed his food. I'm not sure if any of this sounds like your son, or if he might have other things that could point to a sensory issue, but if so there is some occupational therapy that can do wonders. My son couldn't go near sand, grass, hay, fingerpaints, or playdough and after six months to a year of occupational therapy he can do all of those things. Hope this helps, Good Luck and God Bless!