3840 × 2160 vs 7680 × 4320
4K UHD against 8K UHD, side by side.
7680×4320 renders 24,883,200 more pixels than 3840×2160 — 300% more. Both are 16:9 displays, so content fits identically — the difference is pure pixel density. At 8K UHD's pixel count you'll typically run OS display scaling above 100% for readable text.
8K is four 4K panels' worth of pixels chasing content that barely exists. Almost nothing streams in 8K, no mainstream GPU games at it natively, and at monitor sizes the extra density is beyond what eyes resolve at normal distance. Unless you're future-proofing a very large TV, 4K remains the practical ceiling.