> Unfortunately I cannot see how from raid1 of 2*1TB disks migrate to
> raid1(raid10?) of 4*1TB disks
I don't think you can reshape a RAID 1 to a RAID 10.
mdadm can reshape RAID 1/5/6. You can move from RAID 5 to 6 or the other way around.
*Maybe* you can even reshape from 1 to RAID 5/6.
I see these options:
* create a second RAID 1 with the new devices, make this a pv and add it to your vg
* backup all data, create a RAID 10/5/6 with all four devices
* Test if you can reshape from RAID 1 to 5/6
The last option only if RAID 5/6 serves your needs.
Regards,
Jens