At the age of four, he is just old enough to realize that he is ill. Having a fever really saps the strength out of you. He is probably not eating well, because of the throat pain. He does not feel well, and most likely just doesn't want to get up. And when he does, he hangs the arms loosely and sort of staggers. He is small enough that you probably can carry him around, so why walk?
If you have any concerns, though, you should call his pediatrician to ask questions about things. However, if he truly does have numbness, then he should be seen by his pediatrician. Call the doctor, explain the situation, and see if he/she can see the young man on a same day basis.