Originally I had boot on 2 drives raid1, with 2 more drives being hotspares. Then I realized that this makes no sense, if you can just set up a raid1 with 4 drives (where each drive is a copy of each other). Didn't really know if it was supported, but looks like it works.
The entire use all four disks for /boot makes no sense if two disks belonging to the same mirror for the lvm go down. Please stop this nonsense about surviving everything to no benefit. You can have three disks fail and still have a working /boot. For what?