Again, this is up to netflix and amazon in this case, not the blame on either console.
To the consumer, this is of course transparent and if you don't understand what is going on this is just a feature missing that is existing on a different device.
Edit:
Also:
http://www.windowscentral.com/amazon-video-quietly-rolling-out-new-app-xbox-one
https://www.reddit.com/r/xboxone/comments/5xhit5/state_of_4k_hdr_on_the_xbox_one_s/
I think the point is that all the good 4K TVs have reached the processing power and software development to be actually deserving of the title "smart TV". This isn't like a few years back, with shitty apps that barely work, not supporting several formats, being all buggy and slow loading etc.
Plex, Netflix, Youtube etc. are all easier to use (and supporting the new standards, codecs etc.) on my LG for instance. They are treated like channels so you can effortless switch between them. It also keeps at least one app suspended etc. The TVs also connect easily now to PCs (even wireless display sharing or direct content streaming) as well as feature USB 3.0 ports for HDDs etc. to function as media-PCs essentially.
In my case, I completely stopped using Plex on PS4 because the TV's Plex app is just so much more convenient to use.
This means ALL the 4K-related software/app stuff that both the PS4 and XB1 are pushing, is irrelevant. They may catch up feature-wise but then there's still no point of using apps on a second device when you can just switch the "TV" channel instead.
The ONLY 4K-related content that you can not get via your TV itself are UHD Blu-rays - ironically also the only media that fully shows off said TV. It's thus the only relevant feature to put in your "4K" console apart from games of course (which are not 4K anyway but at least HDR makes up for it on both base and pro PS4s).