Hot spare or extra parity drive in RAID array

What is the downtime tolerance of the array? Is it physically close, or in a remote data center? Bascially, if you can tolerate it, a cold spare allows you to do RAID10. The spare is sitting close by, but you have to physically do the swap. If that is not an acceptable scenario, then RAID5 with a hot spare is the only answer left.

Since you already have two RAID1 sets that have 1 drive failure tolerance, you really gain nothing by going RAID10 with no hot spare. Your entire array can still only survive a single drive failure.

That behavior (using a spare) should really be invisible to you; what it's saying there is that it's going to do some drive building trickery, and instead of sweeping the disks and building the parity channel up during the build (slow!), it's going to build the array on three devices and throw the last device in after the fact, to do the parity build in the background.

Your suspicion about the problem with the command as you have it is dead on. If you run that, it'll generate a 3-disk array on 2 disks, then add the third disk from being the temporary hot-spare into the array.. leaving you with, still, a hot spare at the end of it.

You'll want this, instead; mdadm will still do its trickery with the hot-spare, but it'll be invisible to you (you'll see in /proc/mdstat after the creation, that it'll immediately jump into the process of loading data onto the 4th disk):

mdadm --create --verbose /dev/md0 --level=5 --raid-devices=4 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1