I worked in infant toddler centers with children 6 months to 3 years. For these younger children, especially those who might have some awareness of pain in their mouths because of teeth, we gave them frozen "teethers" to hold in their mouths and to bite on and told them, "These are for biting on, we don't bite on people. It hurts people"
I do not recommend biting back especially if it is not right at the moment. Sometimes the bitten child will bite back, but this is rare. Often the children will avoid being close to a child who is apt to bite. Sometimes it appears to become a habit for no apparent reason and totally without warning.
I wonder how verbal your four year old is. And other ways he uses his mouth and teeth. It seemed to me that sometimes this is what certain children go through just before they are able to quickly and clearly express some kind of frustration or strong feeling verbally so they just sum it up with a clenching of teeth on or at the person who is handiest.
I think a four year old should be able to become empathetic and not want to hurt others or make them not like him. I probably talked too much with my children, but I still advise communicating with him about this especially if it goes on for a long time with no improvement.
Could he be worried or angry at you for abandoning him all day long? Has something happened at home or at day care unrelated to the children he bites? Does he bite certain children or just whoever is handy? Have the teachers noticed what happened just before the biting?
Good luck with this It can just be a stage and be over quickly without ever knowing why, or it can be indicative of something you have to address in other ways. I have to admit that the youngest of my five children did bite people including me for a short while. I thought in his case that it was because he was a very quick and physically active child who couldn't get his words out as fast as he wanted. One time he bit me while hugging me as though he just loved me so much he wanted to take a bite out of me. He was not angry, but was being affectionate.
Blessings on you and your son. Hope this can help a bit
N.