In order to scale a 2560x1440 image to 3840x2160, the scaling factor is x1.5. This means that a single pixel in the lower-resolution original image gets mapped onto one and a half pixels, which increases blurriness. This is vastly different to a 4K display running with 1920x1080 input, where each pixel simply gets doubled in width and height, so a 1:1 mapping exists and everything stays sharp.
When looking closely, visual quality differences could appear in text, which does get smoothed by all modern operating systems, though, so it comes with some inherent blurriness anyway. Media playback and gaming shouldn't be affected in any noticeable way. One potential method to detect such a monitor is to look for the pixel size specification, which should be around 0.23 mm. For a 4K panel that number is 0.16 mm, so if the specification of your 1440p monitor lists that number, it probably comes with a 4K panel.
Bookmarks