I ll just go ahead with a fresh v4 install after clonezilla backup.
I wanted to go with ZFS back then (5 years ago or so) but it seemed quite unstable / not well supported.
My main concern now using Raid 5 with 4 disks is the following :
- 1 Disk failure followed by 10To resync creating a 2nd disk failure then I m fucked
Migrating Raid 5 to ZFS might be a side project in a near future.