BTW, software RAID-5 seems scary.  search for threads about UPS and
read/writeback.  I'd get two x 750 and RAID-1 the whole thing.

  Are there any peculiarities with RAID1 mirroring across all 4 disks
  instead of just 2?

That's possible; everyone else just uses 2.  My 3.1 raidframe system is
2 x 400G with the entire disk RAID1.

But, the book code just sees a partition of type RAID, and skips 64
sectors, so it won't know how many copies.

Can you explain your partitioning in more detail?  I would expect (on
each disk) that you have a single fdisk partition of type 169 for the
whole disk (63-$), and within that a NetBSD disklabel (with d 0-$, c
63-$).  I'd expect a (63-1G or so), and e (after a to end).  Both a and
e would be type RAID.  Within raid0, a disklabel with only partition a,
from 0-$.  Within raid1, a disklabel with only b, e, f, ..., starting at
0, continguous.  raid0 would be autoconfigured with root, raid1
autoconfigured without root.  Or perhaps swap is in the RAID-1.

I haven't read the boot code.  See

  src/sys/arch/i386/stand/bootxx/boot1.c

search for RF_PROTECTED.