Adding Drives to RAID 1

In my EX4100, I currently have two 4tb drives set up in RAID 1. I’ve ordered two more 4tb drives to double the size of my array and I have a couple questions. First, how do I go about adding the drives? Second, since the original drives were purchased at the same time, would it be a good idea to replace one of them with one of the new ones first? Then could I pair the the one I replace with the other new one? I’m planning to keep it in RAID 1.

Also looking for the answer to this one. Please let me know if you figure it out John. I’m hoping to do the same thing, upgrading from 2x 8tb in raid1 and adding 2 additional 8tb into the remaining 2 slots.