A new study published in Nature by University of Cambridge researchers just dropped a pixelated bomb on the entire Ultra-HD market, but as anyone with myopia can tell you, if you take your glasses off, even SD still looks pretty good :)
For me it's mostly for text and content with fine lines where the difference is the most obvious. I use a 1440p monitor at work and a 4k one at home. Text is noticeably sharper and easier to read on my 4k monitor and remains readable at smaller font sizes.