Hi
I am fairly new, but had to post as I am in a very similar position. My son is also four and has just started Reception. Lunch time is also a stressful experience for him. He dislikes the noise and is too shy to ask the lunch time supervisors to open his drink. On a number of occasions, nothing has been eaten or drunk.
I am on the point of discussing things with the class teacher, as I think that this has gone on long enough
The difference for me is that I am a primary teacher myself, though have never taught Reception. I do know that by half term almost all children have settled in without any special treatment. However, should any child be seriously unhappy at lunchtime we would do everything we could to rectify the situation.
Please have another chat with the teacher, she may not fully understand how unhappy your child is (most teachers will not be in the hall with the children).
Suggestions I would make as a teacher might be: a friendship group with older, sensible children; her being introduced to a specific lunchtime supervisor who she can use as a point of contact; teacher or TA settling her in the dinner hall.
As a parent I am currently trialling a treat each day in the lunchbox, talking about how kind dinner ladies are and trips to McDonalds to try to de sensitise to the noise!