Why are Pinterest outbound clicks much higher than the Pinterest sessions shown in GA4?

That gap is normal because you’re comparing an outbound-click count in Pinterest with a session count in GA4. Pinterest records an outbound click when someone takes an action that sends them off Pinterest; GA4 only starts a session when it receives measurement and there isn’t already an active session. So multiple Pin clicks can still become one GA4 session if the same visitor comes back before the session times out, and a click doesn’t automatically guarantee a measurable landing-page session. Before treating it as a tracking problem, compare the same date range and make sure Pinterest is filtered to the same destination and organic/paid slice you’re using in GA4. Then use tagged Pin URLs and GA4’s Traffic acquisition report with session-scoped source/medium. The totals still won’t be identical, but the attribution will be much easier to trust.