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The difference you’re seeing is normal, because Google Ads (via GTM tracking) and GA4 don’t measure conversions the same way:
**Google Ads (GTM tag as primary):** Uses last Google Ads click attribution (it gives full credit to the last ad click). Can include modeled conversions (like cross-device) that GA4 may not show.
**GA4 (set as secondary):** Uses data-driven attribution across all channels, not just Google Ads.
Why they don’t match: Different attribution windows, attribution models, and counting methods. Ads = ad-centric, GA4 = site/business-centric.
**Best practice:**
Keep your GTM conversion as the primary in Google Ads. That way bidding uses the signal Google expects (fast, Ads-optimized, consistent).
Use GA4 as secondary for reporting/comparison, and rely on GA4/backend numbers to calculate CAC, ROAS, etc. for business decisions.
So: stick with GTM as primary, GA4 as secondary.
