Migrate from Google Analytics 4 to PostHog: Step-by-Step Guide

Why migrate

PostHog is what you switch to when GA4 has been the wrong tool all along. It’s not designed for “what is my organic traffic” questions — it’s designed for “where do users drop off in onboarding, what feature did the churned cohort never use, and which experiment variant won”. If your real questions are product-shaped, PostHog is a better tool. If they’re marketing-shaped, look at Matomo or Plausible.

Pre-migration checklist

1. Confirm fit. If your top GA4 reports are all about traffic source / channel / campaign, PostHog is the wrong move. PostHog excels at user-level product behavior, not anonymous marketing-funnel analytics.
2. Decide hosting. PostHog Cloud (US/EU) starts free, scales to $450+/mo. Self-host (Helm chart, Docker compose) is mature but heavy — needs Postgres, ClickHouse, Redis, Kafka.
3. Plan identification. PostHog needs posthog.identify(distinct_id) at the right moments. GA4 doesn’t expose stable user IDs by default; you have to wire this yourself.
4. Inventory current product events. If you don’t have product events in GA4 today, the migration is really a “start tracking product events” project. Budget accordingly.
5. Plan retention period. PostHog defaults to 1-year event retention on paid; longer needs custom config or self-host.

Step-by-step

1. Audit (Days 1–4)

Categorize your GA4 events: marketing-funnel (page_view, source/medium) vs product (sign_up, feature_used, churned). Only the product events should migrate to PostHog. Keep marketing analytics on Plausible / Matomo or your ad platforms.

2. Install PostHog (Days 4–6)

<br />
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posthog.init('phc_XXXXX', { api_host: 'https://eu.i.posthog.com' });
[/ma_code]



<h3>3. Identify users (Days 6–8)</h3>
<p>This is the work GA4 hides from you. Call <code>posthog.identify()</code> at sign-up, login, and anywhere you have a stable user ID. Without identification, PostHog is just GA4 with extra steps.</p>
[ma_code lang="js" title="PostHog identify pattern"]<br />
// On login or session restore<br />
posthog.identify(user.id, {<br />
  email: user.email,<br />
  plan: user.plan,<br />
  signed_up_at: user.created_at<br />
});</p>
<p>// On logout<br />
posthog.reset();<br />

4. Map events (Days 8–12)

Each GA4 product event becomes a PostHog capture. PostHog accepts arbitrary event names — no GA4 reserved-event constraints.

<br />
// GA4<br />
gtag(‘event’, ‘feature_used’, { feature: ‘export’, plan: ‘pro’ });</p>
<p>// PostHog<br />
posthog.capture(‘feature_used’, { feature: ‘export’, plan: ‘pro’ });</p>
<p>// Cohort analysis comes for free in PostHog Insights — no GA4-equivalent.<br />

5. Wire feature flags + experiments (Days 12–18)

This is why most teams migrate. PostHog’s feature flag API replaces LaunchDarkly / Optimizely:

<br />
if (posthog.isFeatureEnabled(‘new-checkout’)) {<br />
  // ship the variant<br />
}<br />

6. Dual-run + cutover (Days 18–28)

Run PostHog alongside GA4 for 14-30 days. Build the 3-5 funnels and cohorts you care about most. When PostHog answers product questions faster than GA4, drop GA4.

Common gotchas

• Event volume costs. PostHog Cloud bills per event. Ship $event_filter or sampling early or you’ll get a surprise bill.
• Self-host is heavy. ClickHouse + Kafka + Postgres + Redis + Zookeeper. Don’t self-host unless you have a sysadmin who wants this.
• Session replay is power + risk. It records DOM mutations. Mask PII fields aggressively or you’ll have a GDPR incident.
• Identification gaps. Anonymous → identified merge can lose data. Test the merge path with synthetic users.