How do I reset my iPhone and still have the 11.4 software still on it
I got an new iphone but I’m trying to redownload backup but it says 11.4 software required.
iOS 11.4 has not been released yet.
So it sounds like you've done something relative to beta software that you should not have done.
This is about all we can tell you here, because, as you should realize, beta software cannot be discussed in a public forum.
How to install or uninstall iOS beta software - Apple Support