Doodledog
*Once children are at school we wouldn't think it was normal for children to be running round naked from the waist down, it is normal to wear clothes that cover us.*
But nobody is talking about children being naked from the waist down! The OP is about girls not showing their knickers. Which are already covering their bodies.
The very term 'modesty shorts' says it all, surely? It is about the idea that it is 'immodest' to show certain parts of the body - the sex parts. In this case it is taking that a step further (but only for girls). Not only should they not show their bodies (as per custom and for their own protection), but they should not show the clothing that covers them either.
I am not saying that girls should show their private areas - as I said in the first post that you quoted, I think that they should have agency to wear what is comfortable, rather than wear a dress that then needs to have adaptations to ensure 'modesty'.
Four year old girls are unlikely to be bothered about who sees what (unless they have been brought up to feel that way), so of course it is about the attitude/reaction of the adults who can see them.
The point you don't seem to understand is that very often the knickers that are sold for girls don't always cover the area. If it was covered there wouldn't be an issue.
There is also the issue that we aren't just talking about 4 year olds, some girls at primary school will be 11 and already having periods. You might think it doesn't matter, many adults in school think it does.