3840 × 2160 vs 5120 × 2880
4K UHD against 5K, side by side.
5120×2880 renders 6,451,200 more pixels than 3840×2160 — 78% more. Both are 16:9 displays, so content fits identically — the difference is pure pixel density. At 5K's pixel count you'll typically run OS display scaling above 100% for readable text.
5K's real argument is scaling math: it's exactly double 2560×1440 in each direction, so macOS renders a pixel-perfect 2× desktop where 27-inch 4K needs blurrier fractional scaling. Windows handles fractional scaling better, making 4K the pragmatic pick there. 5K also fits a full 4K frame with editing tools around it.