Create a mdadm raid mirror of an existing lvm

Create a mdadm raid mirror of an existing lvm

Hello I'm trying to figure out how I can create a pretty strange mdadm
mirror of my lvm.
I have 4 disks, (sda,b,c,d). Each disk has 2 partitions: 1Gb - physical
volume 500mb - Swap
My system lvm volume is currently made of two physical drives:
/dev/sda1 - 1Gb
/dev/sdb1 - 1Gb
Creating a 2Gb volume group for my root partition.
/dev/mapper/sys-root
What I want to do now is mirror my sys-root volume onto the other two
disks. I originally thought I would create mirror as:
mdadm --create /dev/md0 --level=1 --raid-disks=4 missing missing
/dev/sdc1 /dev/sdd1
Except I realise that all that would do is mirror the the contents of 1
drive onto the other 3.
What would be the method of mirroring /dev/mapper/sys-root onto my other 2
disks? Unfortunately doing this from scratch isn't an option as I need to
use the existing file system.