Hi,
Well, knowing the code behind all this, I don't see how changing the order of the fields in the interface would lead to a 404 error. Also, it seems that you're saying that it led to a 404 error on the product page on the frontend ? That's even more remote.
I think that both things are not linked. There must be something else at play here.
A 404 error is too generic for my to say anything useful though. I would need the full error with the "debug" setting activated and the URL of the page.
Now, one alternative to moving the fields around is to click on the button to disable the form customization in the green area at the bottom of the product page. This way, you'll get the default position of the fields on the product form page.