No big deal, I just thought I completely missed the boat on your goals

What I use for a high performance filesystem on Linux is ZFS on Linux. I use this for my host OS (and Docker containers) and virtualization platform (Proxmox). The mix of ZFS for all of my fast media and SnapRAID (UnRAID in your case) for my bulk media has worked really well for me.
You will just need to determine if you'd like to use a RAID10 or RAID5 setup with your 4 SSDs (I think that was your plan from the other thread). With spinning disks, RAID5 on ZFS sucks for VMs because your are limited to the IOPS of a single disk, but for SSD platforms, that's not so much of an issue. So, you will just need to determine if you'd like to have the true RAID10 speeds and benefits of not using parity at all or go RAID5 and be able to use an extra 480GB of storage space on your SSD array.