Most of the mobile manufacturers have their own customized Android operating system. Likewise, Asus has its own Android operating system called ZenUI. All their Android smartphones are preloaded with ZenUI as default. Asus has announced the newer version ZenUI 4.0 alongside with Zenfone 4 series launch with a lot of improvements, features, and fresh look. Also, This new version has less number of preloaded applications. Asus has confirmed that the ZenUI 4.0 will be rolled out to Zenfone 3 series as well.
As said, Asus has begun the ZenUI 4.0 to Zenfone 3 series starting with ZenFone 3 Zoom [ZE553KL]. The system update will be rolled out globally in one week. You can check it manually on your Zenfone 3 Zoom device by navigating to Settings > About > System Update.
ZenUI 4.0 Video: