You Can't Downgrade from iOS 11.4
If you can't stand iOS 11.4's performance on your iPhone 7 or iPhone 7 Plus, you'll have to find a manual fix.
Apple's no longer signing off on iOS 11.3.1 which means you can no longer use a loophole to downgrade your iPhone 7 if iOS 11.4 gives you problems.
Unfortunately, earlier versions of iOS 11 are also inaccessible. You can no longer downgrade your iPhone 7 or iPhone 7 Plus to iOS 11.3, iOS 11.2.6, iOS 11.2.5, iOS 11.2.2, iOS 11.2, iOS 11.2.1, iOS 11.1.2, iOS 11.1.1, iOS 11.1, iOS 11.0.3, iOS 11.0.2, iOS 11.0.1, iOS 11.0, or iOS 10.
With the iOS 10.3.3 downgrade loophole closed, there's no way to get off iOS 11. If you move your iPhone 7 or iPhone 7 Plus from iOS 10 to iOS 11, you're stuck with iOS 11.4.
The iOS downgrade process is pretty straightforward and once you get the hang of it, you'll be ready whenever Apple releases a new version of iOS 11.