Who watches their photos on a 4K display and pixel-peeps that they're 20% less that its full resolution? (4K is about 8MP -- but you need to go to 2MP (square root of 8MP) to have half the visual resolution. 5-6MP to 8MP is much closer that it looks on paper, as the change is not linear (pixels cover an area, not a line).
That is bit borderline claim. My reading would be that the image would have about 180 PPI (or less..), which is maybe passable but certainly not above human ability to notice improvement to something like 300 PPI. Assuming of course sufficiently good printer etc.