Do subdomains need GA4 cross-domain measurement if they already use the same web stream?

Usually, no. If both subdomains use the same GA4 web stream and the default cookie setup, the Google tag sets cookies at the highest practical domain level, so a trip from `www.example.com` to `shop.example.com` can stay with the same user and session without cross-domain linking. I'd first check that both sites use the exact same measurement ID and that nobody has overridden `cookie_domain`. The exception is when the subdomains end up using different cookie domains or you see self-referrals/session splits. Google specifically calls that a reason to configure cross-domain measurement for subdomains. So don't add it just because there are subdomains; test the handoff and fix it if the reporting shows a break.