Amazon 4K UHD video purchased online won't be 4K on my iMac 4.5 K Retina retina display!

I was going to purchase a 4k video on Amazon and I received the following message: " Resolution support: 4K UHD videos aren't supported on your device. This video will play in the highest supported resolution"

My device is a new 2021 iMac 24'' with 4.5 K retina display. How can that be? How can I make sure I can watch full resolution in 4K?