2560 × 1440 vs 3840 × 2160
QHD 1440p against 4K UHD, side by side.
3840×2160 renders 4,608,000 more pixels than 2560×1440 — 125% more. Both are 16:9 displays, so content fits identically — the difference is pure pixel density. At 4K UHD's pixel count you'll typically run OS display scaling above 100% for readable text.