It's about shooting, not necessarily the end display.
That 'argument' is asinine. If anybody was to suggest you need to shoot with a medium format digital sized sensor to use a crop camera sensor area for your image, or shoot with a 100mm when you need a 400mm you would be, rightly, laughed at.
Sure a little amount of 'cropping' space to straighten an image etc is nice, but 400% is crazy.
400%? Where'd you get that number? From full-HD it's 100% (factor of 2). It's quite common for me to consume 40% in the process of post stabilization so 100% isn't crazy at all.
FHD 1920x1080 = 2,073,600, 4K 3840 x 2160 = 8,294,400
8,294,400/2,073,600 = 4.
4K is 400% the size of FHD.
Crop cameras have a multiplication factor of 1.6 (for Canon) but a FF sensor is 2.56 times the size of a crop camera's. Or, twice as long and twice as tall, for four times the area. Now before we get into an argument about factors and multiplication, which I am not interested in doing with you, it takes four FHD sensors to make one 4K sensor, if you crop a 4K image to 25% it's area you get the resolution of a FHD 1080 image.
* that was the very first word you typed to me in your first reply to a post of mine in the forum.
Cropping is a linear measure, not an areal one. This is true in both sensor sizes and in post processing software.
4k/2k = 2 = 100% increase or crop of 50% of the image.