using lvm to create vaolume with disk, size, spanned, parition
You need to use LVM (Logical Volume Manager)
First of all , you must be aware that if any of the physical disk fail , the Big 4TB volume will fail too. Backup your data first!
Basically , all you need to do is to partition your data (/dev/sda2 and /dev/sdb1) partition in lvm format then :
There are dozen of sites with howtos lvm like this one.
Lvm is a lot more than these 4 commands , read the fine manual if you want advanced configuration. I hope it will help you