In reality, given your viewing distance and the size of the pixels in question, you basically cannot see the lines between them on a retina display. And having 4 pixels (or 9 in the case of 3x retina on the iPhones X and XS), allows for more fine details to come through, and that text comes across more clearly.
The advantage for the consumer, especially on the Mac, is that the system can display non-optimal resolutions more clearly. Yes, they're still some blurring, but it's not nearly as bad as it has been in the past. And if nothing else, you can use 3rd party software to run the panels at their full resolution.
4K (resolution) is a concept that emerged in the era of digital cinema. In film, the screen resolution is measured by the length of the frame, horizontally, and not vertically, as in television. Since in digital cinema there are different standards for the aspect ratio of the screen, it is more convenient to focus on the horizontal resolution, which remains constant, unlike the height of the frame. But technologies are rapidly developing, and now the video of this quality can be broadcast continuously.