The macOS Catalina 10.15.4 Supplemental Update can be downloaded from the Mac App Store for free using the Update feature in the System Preferences app.
The new software introduces a fix that caused Mac computers running macOS Catalina 10.15.4 to be unable to participate in FaceTime calls with devices running iOS 9.3.6 and earlier or OS X El Capital 10.11.6 and earlier.
Apple says it also addresses an issue with Office 365 accounts, fixes a bug that could cause a USB-C port to become unresponsive, and addresses a problem that caused the 2020 MacBook Air to hang during Setup Assistant or when connecting or disconnecting a 4K or 5K display. Full release notes for the update are below:
macOS Catalina 10.15.4 supplemental update improves the stability and security of your Mac.
– Fixes an issue where Mac computers running macOS Catalina 10.15.4 could not participate in FaceTime calls with devices running iOS 9.3.6 and earlier or OS X El Capitan 10.11.6 and earlier
– Resolves an issue where you may repeatedly receive a password prompt for an Office 365 account
– Fixes an issue where MacBook Air (Retina, 13-inch, 2020) may hang in Setup Assistant or when disconnecting and reconnecting a 4K or 5K external display
– Resolves an issue where a USB-C port in your Mac may become unresponsive
macOS Catalina 10.15.4 could be one of the last updates to the macOS Catalina operating system as Apple transitions to working on macOS 10.16, which is unnamed as of yet.