Depending on whether you update through the App Store, or through a standalone installer, the macOS Sierra 10.12.4 update weighs in at more than 1.5 GB, and represents about a third of the whole of macOS.
Apple does now provide a little more information about some of the new features and fixes included in the update, but that is only a very small fraction of what has changed. Moreover, Apple no longer changes version numbers every time that an app is updated, which can lead to confusion. Here are some lists to help.
Some Mac models undergo an EFI firmware update during the 10.12.4 update. New MacBook Pro models with a Touch Bar also seem to undergo a firmware update for the Touch Bar; the latter can sometimes crash, leaving it unstable: further details are in the list of bugs for 10.12.4.
Applications:
- App Store.app (version 2.2)
- Automator.app, including components
- Calculator.app
- Calendar.app
- Chess.app
- Contacts.app, including plugins
- DVD Player.app
- Dashboard.app
- Dictionary.app
- FaceTime.app
- Font Book.app
- iBooks.app
- Image Capture.app
- Launchpad.app
- Mail.app (version 10.3)
- Maps.app (version 2.0)
- Messages.app
- Mission Control.app
- Notes.app
- Photo Booth.app
- Photos.app (version 2.0), including its print products and Photo Library Migration Utility.app
- Preview.app (version 9.0)
- QuickTime Player.app
- Reminders.app
- Safari.app (version 10.1)
- Siri.app
- Stickies.app
- System Preferences.app
- TextEdit.app
- Time Machine.app.
Applications/Utilities:
- Activity Monitor.app
- AirPort Utility.app
- Audio MIDI Setup.app
- Bluetooth File Exchange.app
- Boot Camp Assistant.app
- ColorSync Utility.app
- Console.app (still version 1.0) – its interface has been improved to make it easier to add and manipulate filters
- Digital Color Meter.app
- Disk Utility.app
- Grab.app
- Grapher.app
- Keychain Access.app
- Migration Assistant.app
- Script Editor.app
- System Information.app
- Terminal.app
- VoiceOver Utility.app.
Core Services with substantial updates include:
- Directory Utility.app
- System Image Utility.app
- Certificate Assistant.app
- Finder.app
- HelpViewer.app
- Installer.app
- ManagedClient.app
- RemoteManagement’s VNC Server and Screensharing
- Setup Assistant.app
- Siri.app.
Most system extensions (KEXTs) are replaced with new versions, as are most file systems, and most Preference Panes, and ScreenReader Braille drivers. Most of the voices for speech are updated, as is a lot of SystemProfiler.
For those using command tools, most of those in /bin, /sbin, /usr/bin, and /usr/sbin are replaced with new versions.
Inevitably, there are also many libraries and frameworks which have been updated too.
(Updated 31 March 2017.)