GA4 vs PostHog: web analytics vs product analytics
One tracks how people find you, the other tracks what they do once inside. The honest comparison, plus the case where running both is the right answer.
GA4 and PostHog answer different questions. GA4 is web analytics: which channels and campaigns bring visitors who convert. PostHog is product analytics: what users actually do inside your product, with session replay, funnels, and feature flags. Its first million events per month are free. Many SaaS teams run both, deliberately.
The short answer
This is the comparison people ask for and the category error people make. GA4 and PostHog overlap maybe thirty percent, and each is bad at the other's core job.
GA4 exists to answer marketing questions: which channels, campaigns, and keywords bring visitors who convert, and how to feed that back into Google Ads. PostHog exists to answer product questions: where users get stuck, which features retain them, what happened in the session right before they churned.
If your revenue lives on a website (e-commerce, lead gen), GA4 or one of its alternatives is your center of gravity. If your revenue lives inside a product (SaaS, app), PostHog-shaped tooling is, and GA4 alone will leave your product team guessing.
What GA4 actually is
Google's free web analytics: acquisition reporting, conversion tracking, audiences, and a tight loop with Google Ads, plus raw export to BigQuery. Free at any traffic volume.
Its real strengths are the ad-platform integration and the price. Its real weaknesses for product work: it is event-based in name but campaign-shaped in soul. Reconstructing "what did this cohort of users do across their first two weeks" means custom dimensions, BigQuery, and patience. Nobody's product team lives in GA4 by choice.
It also carries the EU baggage: consent required before load, and data processed by Google under a transfer framework that is currently valid and currently being challenged in court. The full story is in the GA4 vs Matomo comparison.
What PostHog actually is
Product analytics built for engineers: event capture (including autocapture), funnels, retention and cohort analysis, session replay, feature flags, and A/B testing in one platform. You can self-host the open-source version or use their cloud, which offers an EU region for data residency.
The pricing model is usage-based with a generous floor: the first 1,000,000 events and the first 5,000 session recordings each month are free, then you pay per unit at rates that fall with volume. A small SaaS can genuinely run on the free tier for a long time; a high-traffic marketing site pumping every pageview in as an event cannot, so instrument deliberately.
Its honest weakness is the mirror of GA4's: marketing attribution. Channel reporting exists but is not the point, there is no ad-platform bidding loop, and your agency will not know what to do with it.
Where people pick the wrong one
The two failure modes I keep finding in audits:
- A SaaS trying to run product analytics in GA4. Someone set up "engagement tracking" in GA4 two years ago. No product decision has ever used it, because the questions that matter (activation, retention, feature adoption) are a fight to even express there. The team stopped looking, which is worse than not tracking.
- A store or content site adopting PostHog because engineers liked it. Fair, the tooling is likable. But the business runs on "which channel deserves next month's budget," and now marketing exports CSVs into spreadsheets to answer what GA4 would have answered natively, while the ad platforms bid on thin data.
Both are the same mistake: choosing the tool before naming the questions the business actually needs answered. That naming step is where I start every Tracking Foundation build, because it decides everything downstream.
Can you run both?
Yes, and for SaaS it is often the right answer rather than a compromise: GA4 (or a lightweight, cookieless alternative) on the marketing site for acquisition, PostHog behind the login for product behavior. The free tiers make the combination close to costless; the real cost is discipline.
Two rules keep it sane. First, one naming convention across both tools, written down, so "signup" means one thing everywhere; I install exactly this in Tracking Foundation builds. Second, one owner for the seam: the handoff event (signup, purchase) must be verified in both tools, at the destination, or the two datasets quietly drift apart and you are back to dashboards that argue.
Which should you pick?
- GA4 if you are e-commerce or lead gen, paid channels matter, and your questions are about traffic and conversion. That is its home turf.
- PostHog if you are a product company and the burning questions are activation, retention, and what users do inside. Its free tier means trying it costs you setup time, not money.
- Both if you are SaaS with real acquisition spend and real product questions. Deliberately, with one naming convention and one owner.
- Neither answers "can I trust my current numbers." If GA4 and your backend disagree by 40%, adding a second tool gives you three opinions. A Clarity audit sorts that out first.