People love to talk about 4K as if it's limited to movies. But truth be told, it takes a lot of effort when filming a picture to preserve that 4K edge. You have to use the right film, the right sensors, the right lenses, the right sort of lighting techniques, the right sort of focus, the right sort of makeup and scenery and costumes to preserve the level of clarity that people have come to expect from "4K." So, when displayed on a retina imac, sometimes it makes a big difference. Sometimes it doesn't.
On the other hand, if a computer is rendering text, it has four subpixels to render the text with all the extra clarity it deserves. The viewer doesn't have to imagine that the grey parts of a letterform are indicitive of more detail. Instead, the viewer sees a more detailed letterform, and all the distracting grey forms that distracted from the ideal of crisp back ink on a crisp white paper have suddenly become a lot smaller, and hard to pick out.
The retina advantage is most apparent when rendering text and simple line drawings. Sure, if you are reading National Geographic, and it's been optimized for the retina display, it looks like freaking National Geographic. It is really nice to look a museum website and see all the additional detail that coarser displays miss out on. But those are professionally photographed using the best technologies available.
In most cases, what you will notice is the text--san serif fonts look clean. serif fonts look elegant. And it will all be easier to read.
