Apple has released the update to Catalina 10.15.4, which is now available through Software Update. It is another substantial download, of around 3.42 GB, and brings with it firmware updates for all models.
This update finally brings iCloud Drive folder sharing, which had originally been intended for release with the first version of Catalina, but was delayed because of all the iCloud problems it brought during beta-testing. You might like to ensure that all your iCloud Drive files are thoroughly backed up before updating to 10.15.4.
There are also improvements in Screen Time which can apply communication limits, and playback control of music videos. Music sees time-synced lyrics. Safari can import Chrome passwords. There is OAuth authentication support for Outlook.com accounts, and several other lesser new features and bug fixes.
One bundled security update is the TCC database, which now becomes version 150.19 (in Catalina 10.15.4).
Security Updates for High Sierra and Mojave 2020-002 are also available now. These are substantial too, at around 2 GB, and bring the same firmware updates.
Full listings of current firmware versions following these updates are provided here for Catalina, and here for those running High Sierra or Mojave. They are also used automatically by my free utility SilentKnight, which automatically checks your firmware against its databases. Thanks to Pico for kindly supplying those lists.