How much is the file size that supports you to record at 4K format video?
ZenFone 3 Deluxe (ZS570KL) / ZenFone 3 Ultra (ZU680KL) / ZenFone 3 Zoom (ZE553KL)：
The 4K video file supports up to 4GB (About 12 minutes) if file is storaged to FAT32 format SD card.
There's no size limitation if you save the 4K video file to internal storage or exFAT format SD card.
ZenFone 3 （ZE552KL / ZE520KL）:
The maximum 4K video file size is 4GB if users save it to external storage such as FAT32 format SD card.
If you need to save 4K video files with exFAT format memory card or internal storage, there's no size limitation after upgrading firmware to 126.96.36.199_M3.7.28 .6_20160823 or latter version.
For how to update firmware on your device, please refer to https://www.asus.com/tw/support/FAQ/1009748 for your information.
Note: When you record the video with quality set as "4K", auto video stabilization will be disabled.