Are you using the new 4 1TB drives to expand the capacity of your existing RAID6 VD or you're creating a seperate virtual disk for the system to access?
Anyways, either both ways, you won't have to reload your OS.
There is no definite answer on the duration of rebuilding/reconstruction of existing disk array, it will be depending on the total space usage. I assume you are using almost all the space available on the RAID6 VD, that's probably the reason you're expanding with another 4. So reconstructing 6TB of data, would probably take some time.
But if you're creating just a new VD as a seperate virtual drive for the OS, then it's instant.
The four 1 TB hard drives will be added to the existing RAID 6 array.
Currently, I'm only using 100 GB for the Server OS within the RAID 6 array.
Then, you would use the Reconfigure disk to add the drives into your existing array. Then use Online Capacity Expansion to expand the array size. Be sure to do full data backup.
What utility do I need to load so I can do this within Windows without rebooting the server?