I had to do something similar for a guy a few years back. He wanted rings sliced from a old oxygen cylinder, why IDK. I used a piece of angle to make a straight down the cylinder about 6x around. Then made a small V rolling jig for the cutting torch that used 4 ball bearings as rollers and held the torch in place. I was a little wasteful on the cut as he couldn't have a rounded pierce so I lit the torch, slid the pipe up to start cutting length wise and then against a hard stop (depth stop) and started rotating. It worked really well. More accurate than my bandsaw would cut.
You could do the same with a plasma. Just use a hard stop, and once you get that set at the correct height each one will turn out the same. Realize your 1st one will probably be scrap as you get a feel for the feed of rotating the pipe.
BTW, the lines were to make sure the cut was going parallel to the faces.
Edit, I'll add, if you want absolute precision, find someone with a large enough lathe that can part them off for you. Although unless you know someone that could get expensive. Rolling 1/4 into that tight of a diameter without a capable machine is going to be comical for anyone watching.
Edit #2 I'm all for the 4.5" versatility, but there is no way in hell I would be attempting to do this with a 4.5" grinder and cutoff wheels. Not at 1/4" wall. Each cut would probably be a few bucks of abrasive disks alone. We're talking 31" of linear cut each time total of ~550"
