I just want to say that I had this same problem installing wordpress network on my localhost using xampp. Everything installed correctly but when I went to add a new site, everything would work except you get that 404 error when trying to view the page.
After hours and hours of searching and testing fixes I finally came across what should have been done correctly the first time. The .htaccess file.
Since wordpress on my localhost didn’t create an .htaccess file, I had to create it myself and when I did it was the wrong file type. So wordpress was not reading the htaccess correctly.
So just make sure you make the right htaccess file before you try anything else. That’s all I needed to do.
