hi S.,
How lucky these children in your care are to have such a caring provider!
I am sure I don't have to tell you that 4 yr olds can be notorious for suddenly becoming 'picky' eaters (I have one). It sounds like there is something else troubling this little guy. Food can be such a power issue, or sometimes separation anxiety can get triggered when it's time to eat.
As far as an eating disorder, in my opinion - no. But since it sounds like he has a serious lack of energy and circles under his eyes something needs to be addressed and changed.
Would you consider loosening your rules about mealtime in order to see him healthy and happy? I would talk to his parents and let them know that he appears to be unhappy with something around mealtime and you need to know what it is - and ask them to promise him you won't be angry or upset with him, but rather will do whatever you can to fix it for him.
My daughter was freaked out over one daycare provider who served spaghettios often. For some reason my 4 yr old is completely grossed out by spaghettios, she got sick on them once and even the sight of the can will cause her to start gagging! (talk about an eating disorder) haha. Anyway this lady was kind in that she didn't force my daughter to eat them, but even when the other kids were eating them she would feel terribly sick and she was very afraid to tell her what was wrong, so my little girl finally told me and I had to talk to the daycare provider. She fixed it by allowing my daughter to sit at a different table when the other kids were having spaghettios - and it was a phase that passed.
Anyway, it might be something just little that is throwing him off. Or, you might have to temporarily give up the no-snack rule without eating 4 bites - maybe that is intimidating him to the point of not being able to eat at all. And just offer healthy snacks instead of junk snacks. I know when you have a daycare everything has to be done by a schedule or you never make it out of the kitchen, but maybe you'd make an exception to get him eating right again. :-)
Everyone will have different opinions, but I do not believe food should be connected with reward of punishment in any form, I just lay out my 4 yr old's plate and either she will eat it or she won't, and if she doesn't I make a healthy snack for later rather than allowing a sugar snack.
best of luck to you,
W.
:-)