There doesn’t appear to be a changelog to accompany the update just yet, then again Android 4.4.4 doesn’t bring any sweeping changes or new features. However, it does contain an important security patch for an OpenSSL bug, which would have allowed for a “man-in-the-middle” attack.
The build number, 14.A.0.108, has currently appeared in Russia, India, and Indonesia, and should be heading out to other regions in the very near future. The same build version has also been spotted at the PTCRB, a certification review board for mobile devices, for use with the Xperia Z1 and Z Ultra, which suggests that Android 4.4.4 should start rolling out for these devices shortly.
The update should arrive in its own time OTA, or you can force a manual check by heading over to Settings > About phone > Software updates > System updates.
Let us know when you get it.