I'd love to play with BTRFS RAID, but I also like to play with whole disk
encryption too. Right now I can take 4 hard drives and combine them into one
RAID block device, install LUKS on that block device and add file systems on
top of that.
The only way to make this work with filesystem RAID is to create 4 separate
encrypted disks and enter 4 passphrases every time the system boots.