Since they are both the same aspect ratio (16:9) you can also take the ratio of one linear dimension and square it to get the area ratio.

So:

(5120/3840)^2 = (2880/2160)^2 = 1.777

4k is 4096 x 2160, 5k is 5120 x 2880

There's also a really helpful diagram on that page

(5120 × 2880) / (4096 × 2160) = 1.6666