The updates fix an issue where some devices running iOS 13.4 or iPadOS 13.4 couldn’t participate in FaceTime calls with devices running iOS 9.3.6 and earlier or macOS El Capitan 10.11.6 and earlier. They also address a bug with the Settings app where choosing a Bluetooth from the quick actions menu on the Home screen would fail.
The iOS and iPadOS 13.4.1 updates are available on all eligible devices over the air in the Settings app. Go to Settings —> General —> Software Update. option to install the update if it is made available to them.