Find a real and I mean REAL 4k movie and watch it. 4k video on YouTube is crap compared to the real thing. Then use 4 different apps at the same time and assuming you're on Windows 10, snap each app to a corner and see how the extra resolution let's you see more than before. Then try to run an older game like vanilla skyrim and see how clean it looks and still get 60fps. Then try a newer game like skyrim special edition and see how clean it looks but push your gpu hard to max with high temps. Then try a demanding game like the witcher 3 and see your gpu get destroyed with max usage, near 100c temps, but getting onky around 25fps. You get so angry you either return the monitor for a 1440p or whip out that credit card and get a gtx 1080ti and call it a day.