After thinking about it, I don't need the additional space right now. I have a little over 7TB of space and since I've been converting the full disc ISO's to movie only MKV's on my server (it's primarily a media server) I've been reclaiming space. I just added 26 movies and didn't touch the 7TB, just used up the reclaimed space (and still have some of that left) and I still have movies and TV shows to convert. When I actually do run out of space I'll decide what size to move up to, do I want to consolidate etc., at that time. Hopefully you'll be around to offer some more of your great advise!
I remember 5 1/4" drives, that's all my first computer had. I remember being so happy when 3 1/2" 1.44MB floppys became available. I think my first hard drive was a Quantum but I don't remember the size.
I'm going to stick with your original advice and just replace disk 4 and hopefully be able to clear it and have an emergency spare.
Thanks again Frank1940, I told you getting me thinking was dangerous lol