ASUS has announced on Twitter that the Android 4.4 KitKat update has started rolling out for the ASUS ZenFone 4. It's a bit late now to do this but better late than never. Meanwhile, those who bought the bigger and more expensive ZenFone 5 and ZenFone 6 are understandably miffed as their phones are still running Jelly Bean. ASUS hasn't commented on when these phones will be getting the KitKat...
Source: http://www.gsmarena.com/asus_releases_android_44_kitkat_update_for_the_zenfone_4-news-9805.php