@eniko this statement needs a qualification in terms of screen size. 4K on a 15" is just scarce of 300dpi, and for text it really does matter: not in the sense that you can fit more in the same screen estate, but in the sense that the legibility is better (yes, the benefits are higher at smaller font sizes, but standard-to-larger sizes also benefit).
A 27" 4K monitor has approximately the same DPI as a FullHD 15.4. Decent, but not particularly good, unless viewed from a significant distance.