There is absolutely no reason to go back and get the tire flipped. When installing a tire, you look for "inside" or "outside" - if it lacks that, you throw it on however you're holding the tire. The fact that the other 3 have the date code showing is likely completely random.
The fact that they're over a year old is also of no concern. I just got some brand new staggered Michelin PS4S for a customer from the warehouse and they were dated Mid-2022. Mainstream tires are usually delivered to me with a 2023 or 2024 date. But specialty EV tires or odd sizes don't move as much inventory, so they sit for a little while at the warehouse.
I don't think Tire Rack starts discounting their tires until they hit 3 or 4+ years old. You can sometimes find their old stock for sale at a nice discount.