Following the release of iOS 13.4.1 last week, Apple has stopped signing iOS 13.4. This means downgrading to the OS on your iPhone or iPad is no longer possible.
iOS 13.4.1 was a relatively minor bug-fixing release aimed at fixing the Bluetooth and FaceTime issues that were inadvertently introduced in the iOS 13.4 update. For most users, Apple not signing iOS 13.4 is not a big deal. However, if you tend to jailbreak your iPhone, it is usually recommended that one stays on an old version of iOS.
Checkra1n has already been updated to support jailbreaking iOS 13.4.1, so owners of iPhone X and older devices can easily jailbreak their device using it. Newer devices like the iPhone XS, iPhone XR, and the iPhone 11/Pro cannot be jailbroken whether they are running iOS 13.4 or iOS 13.4.1 though, since the exploit Unc0ver jailbreak uses for jailbreaking iOS 13.3 was patched in subsequent releases by Apple.
If you were looking to downgrade your iPhone or iPad to iOS 13.4 for any reason whatsoever and had not already done so, there’s not much you can do now. Your iPhone is now stuck running iOS 13.4.1.