EXT4 and ZFS are PARTITION formats, NOT disk formats (if only a single partition is on a disk, I guess you could call it a disk format, but actually a misnomer). EXT4 and ZFS partitions can exist on the same physical disk; just need to use the appropriate command set for which filesystem you/QNAPinternals are manipulating.
If you’re on Hero, look at all the partitions on a disk that’s in use as part of a data pool. The data pool partition will be ZFS, but tell me which filesystem(s) you see on the other ‘hidden’ partition(s)…