What’s the difference in FHD (1080p) and UHD (4k)?

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.
Stay with me for a sec – 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, like shown above, 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 (see example below). If you put a 50″ image on your 50″ TV, then no resizing is done, and no loss of quality.

So the same logic carries over with 1080p to a 4k TV, except 4k is four times larger than 1080p, so you’d lose even more picture quality than the example above.

Here’s a visual comparison. The first is 1080p. The second is a 1080p image sized up to a 4k space. The last is a 4k image in a 4k space.
Notice the quality loss in the middle image compared to the last image?

1080p and 4k are named for their dimensions in pixels (yes, the little dots that make up your screen and images).

1080p means 1920 pixels wide by 1080 pixels tall (the “p” stands for progressive, but that’s not important here). That’s about the equivalent to 6.4” x 3.6”.

4k (also known as UHD or 2160p) means 3840 pixels wide by 2160 pixels tall. They’re rounding 3840 up to 4000. That’s about the equivalent to 12.8” x 7.2”.
(Note: The numbers mentioned in 4k may be double those mentioned in 1080p, but 4k is actually FOUR TIMES the size of 1080p!)

“DCI 4k” or “Cinema 4k” is 4096×2160, but that’s not commonly used with TV/Broadcast.

So what does this mean to you?

Well, it may mean nothing. Many people watch videos on a phone or tablet and may not be able to tell the difference in 1080p and 4k, but 4k is a much clearer image overall.
If you’re watching on a 4k TV, there’s a good chance that you’ve noticed a quality difference when watching older TV shows. That’s because the older show may have been in 720p (yep, smaller than 1080p) or in 1080p and is being sized up to 4k, which results in a loss of 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.