Why four countries and not a hundred and fifty
Plenty of sites advertise salary comparisons for 150 countries. Nobody maintains 150 tax codes accurately, and a plausible-looking wrong tax figure is worse than a smaller set of figures you can verify.
Each country here is encoded from its own tax authority's published rates, linked on every page, and covered by tests that assert the published worked examples rather than whatever the code happens to produce. More countries will be added the same way, one at a time.