Apple has stopped signing iOS 13.4, which means you can no longer downgrade to that software version. It comes after iOS 13.4.1 was released recently, with bug fixes for iPhone and iPad.
iOS 13.4 introduced a handful of major changes, including trackpad and mouse support for iPad, iCloud folder sharing and Universal Purchase support. By unsinging it, Apple intends to prevent jailbreaks on that software release, and keep users on the latest and more secure version of iOS.
Downgrading is generally useful, however, when newer software updates cause issues with a device and reinstalling previous versions of iOS can fix them.