The easy answer is “one is bigger than the other”, but who cares about that if your screen is always the same size, right? Well, kind of.
Your TV may always be the same size physically, but the image you put on it isn’t.
Let’s say you have a decent size TV, say 50″ and you put a 25″ image on it, that image is too small for your TV. To get a visual, see the image below.
But TV makers are smart, they know you won’t want to watch the image small, so they size it up to fill the whole screen.
In doing that, you reduce the quality of the image, meaning it may be pixellated, grainy or even blurry. If you put a 50″ image on your 50″ TV, then no resizing is done, you get 100% picture quality.
So the same logic carries over with 1080p to a 4k TV, only 4k is four times larger than 1080p, so you’d lose even more picture quality.
If you’re thinking “oh, I don’t even have a 4k TV”, keep in mind that there was also a time that you didn’t have a 1080p TV. Studies suggest that in 10 years, 6k TVs will be the standard (yep, 6k is 6 times the size of 1080p!).
After all that technical babble is over, I’ll be real with you. 4k isn’t going to change your life, but watching a lower resolution video on a 4k or 6k TV means the quality is reduced significantly.
Want a more detailed explanation? Check out our “why 4k?” article here.