Ok. I've not had hands-on experience with multiple storage pools (my NAS are 2-bays), but expect you could either grow your current 2 disk storage pool with the new disks or create a new 2 disk storage pool. That was my 2nd question. The screenshot you posted looked like you were creating a new volume.
I thought you were wanting to grow your 2 disk SHR to a 4 disk SHR, hence the questions and hopefully the links can get you sorted out.
There must be something, different filesystems / separating data for a reason like backups.
I spent days unpartitioning my iMac's internal disk and associated backups. It originally came with a 250GB disk so upgrading I kept the boot partition like that and a second 750GB data partition, all because of existing backup disk sizes. Decided to merge it back to a single 1TB SSHD. There's less day-to-day management with one big partition vs shuffling files around.