Does she do it at home too when she is nervous?
Four and already stressed? Or just a bit anxious? I went to my first year of school at age 4, and it was just half day. Is she in school all day or half day?
or it could just be her way of soothing herself- like thumb sucking- Does she seem anxious? Ask her what else she acts like while she is chewing- if she seems fine and it is just soothing her from the anxiety I bet she will outgrow it......
Teachers should keep you updated on this sort of thing but they are not psychologists. Don't overreact needlessly. You may even attend school at the times teacher says this occurs and see for yourself.
The teacher is correct - chewing things is a common behaviour for children suffering from anxiety. Of course, it might be teething issues (but I doubt it). I might suggest you google the phrase "school anxiety" or "chewing on clothes" or similar words/phrases to find out more information about this issue. Children suffering from stress often find relief in this behaviour - the key is to lessening the stress/anxiety in the classroom and perhaps your son's teacher might have some thoughts on this process. Hope this helps.
Are his permanent teeth coming in? It could be that there is pressure in his mouth. My daughter age 4 has been chewing on her hair lately and she is getting her permanent bottom teeth now.