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GA4 has several advantages over UA to account for the evolving privacy landscape. For one, it supports user modeling in ways that are GDPR compliant, and you simply cannot count these users in UA. Apple and the leading browsers have made changes with cookies that have necessitated product changes, but not using fingerprinting as suggested elsewhere.

GA4 also reports on combined app+web data whereas UA did not.

Another distinction is that GA4 has better support for muti-touch attribution, e.g. all of the user ad touchpoints, for ad traffic, whereas UA was primarily last click, or first click if you look at the user acquisition reports.