>>68730691080p to 4k is exactly double the vertical and horizontal pixels (1:2), so each pixel of a 1080p image just uses 4 pixels of the 4k display.
1080p to 1440p isn't a whole number ratio, so you're doing something more like 1:1.3, which means not every pixel of the image gets its own pixel of the display, causing it to look blurrier than it would on a 4k monitor.
It's not about the density, it's about getting the pixels to line up, basically.
