[blue68f100]

Not familar with the 4200. SO I don't know it the OS is stored in flashram.
A simple test would be to set the unit up as JBOD.
Remove all drives and install a 250 in drive 1.
If the OS is stored in flashram it will auto format and install the OS. Then add the other drives.
if not
the OS is stored on the HD, and it uses a std boot loader.

set up raid 1 with drives 1 & 2, after setup complete, fail drive 2, install new, should do the mirror, then fail drive 1, install new, should mirror.
Once the OS is on the main drives just add drives 3 & 4. let it do its thing.

I see from another thread you are using Guardian OS

Raid 5 drives are suppose to be with in 3% capacity of each other. Not a good choice for major upgrades.