The cynical might now be asking why Apple didn’t brand 10.10.4 as 10.10.5 or even 10.11, there are so many changes and fixes.
First it is a complicated update. When the download is complete and ready to install, your Mac will restart. Only instead of doing a normal restart, it should perform a firmware update. This is marked by a short continuous audio tone, following which there is a progress bar on the display. Do not interrupt this in any way, or you could end up taking your Mac to your nearest Genius Bar (or trying the emergency repair detailed here).
Once the firmware has been updated, your Mac will restart a second time, and perform a regular OS X update sequence which will probably take a good ten minutes or so.
The firmware update is in fact Mac EFI Security Update 2015-001, which addresses a couple of significant security issues in the firmware, one of which (‘Rowhammer’) could lead to corruption in some DDR3 memory. Time will tell whether this completely fixes it, but it looks promising.
Among the long list of OS X security fixes are those relating to dylib hijacking, as detailed here and elsewhere, and discovered by Patrick Wardle. There are many other important fixes, making this an essential update, as soon as you can.
Bundled with the update is Safari 8.0.7, and Apple has released 7.1.7 and 6.2.7 for those on older versions of OS X. These contain several important security fixes too.
However, apart from the unannounced firmware update (I wish Apple would tell us what to expect during an update, instead of expecting our blind trust), there are some problems which have already become apparent. Most immediate is that – perhaps pending a separate update shortly – iTunes remains at version 12.1.2, which cannot use Apple Music. If you try to join Music on your Mac before the separate update to iTunes 12.2, you will be told that your version of iTunes is not compatible.
Hopefully iTunes 12.2 will be along any second now.
One potential improvement in 10.10.4 is that the troublesome
discoveryd (seen running in 10.10.3, above), which handled network and Internet connections and was known for causing problems, has been dropped, and OS X has reverted to using mDNSResponder (seen running in 10.10.4 below), as before Yosemite. I outlined previous problems here. Hopefully this will blow those connection problems away.