There are actually two different problems.
Screen estate and pixel density.
The screen estate of a FullHD screen is nothing to scoff at. 1920x1080 points is larger than most laptop screens, it's much larger than the default MacBook desktop of 1440x900, it's wide enough to display basically all websites ever made and all Mac apps ever made, including the heavy ones like IDEs or photo editors.
If you go from a FullHD screen to a larger one (QuadHD or even 4K) without changing your scaling, you're only getting more screen estate, nothing else. The image is not more detailed, sharper or anything. And there are even downsides: I use a huge 43" 4K screen with 100 % scaling, so the screen estate is equal to FOUR FullHD screens, and that's ridiculously too much for a single app, so fullscreen mode goes basically unused on my machine.
Then there's pixel density, or scaling. The amount of pixel different screen elements (fonts, buttons) are drawn with is constant, whether you use a pre-retina MacBook or the crazy 43" panel that I use.
But most OSes out there allow you to choose your own SCALING, that is, they allow you to trade screen estate for more pixels per every button, letter or icon. This makes the elements larger as the usable screen estate shrinks.
Retina Macs used to use 2x scaling (200 % in Windows) but now they default to slightly more than that.
Whether you're going for screen estate or more details depends mainly on how big your screen is, physically.
I COULD make my display look like a FullHD screen in 200 % scaling, but that would rob me of the screen estate I need and enjoy.
So the questions you have to ask yourself are:
Do you feel constrained by the screen estate of a FullHD screen?
Would you pay money to have your letters drawn with more pixels?
If one of the answers is YES, you will probably need a larger screen, maybe a QuadHD one. If both of the answers were YES, well, you'll likely need a 4K or larger screen, so there's enough pixels to give you more screen estate AND more pixels to draw everything with.