Obviously, those who so adamantly think that 4 year olds and even 3 year olds should be able to wipe their own bottoms, then you haven't read the OP properly.
The issue, as has been repeated a few times, is that the OP's son has an issue with cleanliness, he gets in a panic about it and gets very very upset.
It sounds like the nursery not only will have to manage bottom wiping but also a very upset child. Perhaps they are happier to comfort him than wipe his bottom, which seems very mean to me.
reality1 I hope that when you have another discussion with nursery that you will be able to find a way through this.
I am totally screwed with DS1 then as he is 3.4 and not potty trained yet. We tried for a month and he only indicated verbally that he wanted the potty six times. He can only pull his trousers up at the front, has trouble understanding the concept of reaching round the back and pulling the back of his trousers or pants up. He also can't take his own shoes off yet, as he also doesn't seem to understand the concept of pushing the back of his shoe off. There are also questions of autism as he has autistic traits. But, it is really hanging in the balance.
So it seems that if your child is on the cusp of it, then you get very short shrift.
Luckily we live in New Zealand now where they seem to be much more understanding and accommodating with such things, at preschool at least.
Also, people who are talking about the good old days where all children could proficiently wipe their own bums and were potty trained by 3, have you thought that there may have been more accidents then as well?