@NT
Note that all these below applies to MacOS. On Windows there are no scaling issues.
When you scale the UI bigger on a 4K display...
For example in Photoshop, when you view a 4K photo/graphic at 100% zoom, that file doesn't fill up the whole screen which it should. Certain apps will have this issue.
However, if you use Affinity Photo to open the same 4K photo/graphic, that 4K file will fill the screen at 1:1 so you see actual 4K file on a 4K display.
So whether you can see actual 4K with certain files will depend also on the software you use.
For watching 4K videos, no problem, you will see 4K content in actual 4K.
Regardless of all those issues, you'll still see really sharp images in all scenarios.
Sidenote:
Apple choose to make their 27-inch iMac 5K instead of 4K is so that MacOS can scale UI to 2x and there won't be any of those scaling issues mentioned.
@NT
Note that all these below applies to MacOS. On Windows there are no scaling issues.
When you scale the UI bigger on a 4K display...
For example in Photoshop, when you view a 4K photo/graphic at 100% zoom, that file doesn't fill up the whole screen which it should. Certain apps will have this issue.
However, if you use Affinity Photo to open the same 4K photo/graphic, that 4K file will fill the screen at 1:1 so you see actual 4K file on a 4K display.
So whether you can see actual 4K with certain files will depend also on the software you use.
For watching 4K videos, no problem, you will see 4K content in actual 4K.
Regardless of all those issues, you'll still see really sharp images in all scenarios.
Sidenote:
Apple choose to make their 27-inch iMac 5K instead of 4K is so that MacOS can scale UI to 2x and there won't be any of those scaling issues mentioned.