1920 × 1080 vs 3840 × 2160
Full HD 1080p against 4K UHD, side by side.
3840×2160 renders 6,220,800 more pixels than 1920×1080 — 300% 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.