GA4 stops at the front door. Everything worth fixing happens inside
GA4 is very good at telling you who arrived and where from. It goes quiet the moment they land, which is exactly where your conversion problem lives. The layer most marketing stacks are missing, and how to add it without ripping anything out.
Founder of Agentique, where he leads delivery by turning commercial strategy and business analysis into specs the AI engineers build agents against.
For most of the last fifteen years the playbook was settled. Install Google Analytics, watch the traffic line, spend more where it goes up. If sessions were climbing, you were winning.
The job has moved on and the measurement has not. Acquisition is now half of it at best; the other half is what happens in the ninety seconds after the click, which is where almost every conversion problem actually lives. If the only thing you measure is where visitors came from, you are optimising the half of the funnel you can see and guessing at the half that costs you money.
GA4 will tell you a thousand people reached the billing step and four hundred did not finish. It will not tell you the promo field silently rejected every code with a space in it.
That second sentence is not a GA4 failing. It is a category the product was never built for. Which is why the answer is not a migration. It is a second layer.
Two jobs, not two products
Think of the site as a shop. One system stands outside and counts who walks up, where they came from and which poster they saw on the way. A different system stands inside and watches what happens between the door and the till. Asking either one to do the other's job is how teams end up with two dashboards that disagree and no answer from either.
GA4 is the acquisition specialist
Keep it, and keep it for the right reason. GA4 is the sign above the door and the doorman: it connects natively to Google Ads and Search Console, and it is what feeds conversion signal back into Google's bidding models. People forget that second part when they threaten to rip it out. The feedback loop is not a reporting nicety. Starve it and your paid performance degrades in ways no other tool will explain to you.
PostHog is the experience specialist
PostHog is an all-in-one product analytics suite: events, funnels, session replay, heatmaps, experiments, surveys and error tracking behind one script. It knows almost nothing useful about your media plan. It knows a great deal about the ninety seconds GA4 sleeps through.
What the second layer actually buys you
Four things, and they are worth naming individually because each one removes a specific piece of organisational friction rather than adding a chart.
- AutocaptureNo ticketReplacesA developer ticket, a sprint and a deploy per button you want to measureInstall the tag and pageviews, clicks and form interactions are recorded from that moment, retroactively queryable. You still define named events for the handful of things that must be precise. What changes is that the question "can we track this?" stops being a roadmap negotiation and becomes a filter you apply this afternoon.
- Funnels wired to the evidenceThe whyReplacesReading a drop-off percentage and inventing a story to explain itEvery step of a funnel is clickable through to the session replays and heatmaps of the people who fell out of it. You stop arguing about why forty percent abandon at billing and start watching six of them do it. In our experience this is the single change that shortens the distance between a number and a fix.
- Experiments and surveys in the same scriptOne vendorReplacesTwo more subscriptions, two more tags and a third definition of a conversionA/B tests, feature flags and on-site micro-surveys run against the same event stream as the analytics. That matters more than the licence saving: an experiment scored on the same definition of conversion as your funnel is an experiment you can actually act on.
- Scouts that watch so nobody has toOpen betaReplacesSomeone remembering to open the dashboard on a MondayScheduled agents watch your funnels and retention paths and file a report when something moves, and the replays come attached. Worth being precise about the state of it: this is PostHog’s self-driving work, currently in open beta, and it drafts rather than ships. Treat it as an alerting layer with unusually good evidence, not as an autopilot.
Running both without ripping anything out
The good news is that this is not an architecture project. Two scripts, one consent gate, and a decision about which number wins.
| Layer | AcquisitionGoogle Analytics 4 | ExperiencePostHog |
|---|---|---|
| The job | Where the visit came from | What happened during it |
| Who it feeds | Google’s bidding models, Search Console, the media plan | The people deciding what to change on the page this week |
| Unit of analysis | Session and source | Person and journey |
| The question it answers | Which channel earns the next €10k? | Why does the billing step lose four in ten? |
| Instrumentation | Tag manager, conversion events, consent mode | One script and autocapture; named events where precision matters |
| Where it goes quiet | At the click | At the media plan |
Neither column is a subset of the other, which is why the comparison keeps being framed as a choice and keeps not being one.
- 1Leave GA4 aloneKeep the tag, keep the conversion events, keep them flowing into Google Ads. Narrow what you ask of it: acquisition, channel performance and search. Stop using it to answer on-site behaviour questions it was never going to answer well.
- 2Add PostHog for everything after the landingSession replay, funnel analysis, heatmaps, landing-page experiments, form interactions. Start with the one funnel you already argue about rather than a full taxonomy. Autocapture means the historical data is there when you think of the next question.
- 3Decide once which number is the truthDo it before the first time the two disagree, not after. Name the tiebreaker per question: GA4 for channel and spend, PostHog for on-site behaviour and experiments. Then write it down where the team reports from.
The European part
If you operate under GDPR this is not a footnote, it is often the deciding factor. GA4 has spent years in and out of European legal argument over international transfers. The hybrid stack does not make that argument disappear, but it does mean your most granular behavioural data, the replays and the individual journeys, can sit somewhere less contested.
- Frankfurt hosting. PostHog Cloud EU is a managed instance running on servers in Germany. Same product, EU-resident data, no self-hosting burden.
- IP capture off by default. On Cloud EU, IP data collection is disabled by default for new projects, so the compliance headache you would otherwise have to remember to configure is already handled.
- Redaction before storage. Masking in session replay, autocapture controls, and realtime transformations that strip or anonymise properties before the event is written to disk.
None of that is a substitute for consent. It changes what you are asking consent for, and it shortens the conversation with a DPO considerably.
The other thing worth knowing before you build a business case: the free tier is large enough that most marketing sites never leave it.
Free monthly allowances as published on PostHog's pricing page in September 2026, and reset each month. Vendor pricing changes; check it before it goes in a business case.
Where to start
Not with the tool. Start with the sentence you cannot currently finish.
Pick the one drop-off in your funnel that somebody has already explained twice with a theory nobody has tested. That is your first PostHog question. Install the script, let it autocapture for a fortnight, then open ten replays of people who fell out at that exact step and watch all ten without taking notes. The pattern shows up in the first three, and it is almost never the thing anyone theorised.
The point of the second layer is not more dashboards. It is that the argument about why ends, because somebody can now play the tape.
- PostHog pricing and free-tier allowances.
- PostHog documentation on GDPR compliance, EU Cloud hosting and PII redaction.
- PostHog documentation on self-driving and Scouts (open beta).
- Google Ads Help: create Google Ads conversions based on Google Analytics key events, the import that feeds Smart Bidding.
We set this stack up with marketing teams and train them to read it, on your own funnels.
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