GA4, or Google Analytics 4, is Google's free analytics tool that measures what people do on your website and in your app, as events: a page view, a click, a form submission, a purchase. It tells you where your visitors come from and what they do once they arrive. It then passes to Google Ads the actions your campaigns bid on.

Think of a ship's logbook. Each event is a dated line with its details, and each report is just a way of counting those lines. For an advertiser who opens GA4 and sees numbers that do not match the sales, everything starts here: knowing which lines the logbook counts, and which ones it highlights.

What Is GA4 Used For

The Analytics help center presents GA4 as a type of property that collects website and app data to better understand the customer journey. For an advertiser, it does three jobs.

  • Know where your visits come from. Each session goes into a channel, such as organic search, social networks, email or AI assistant, based on rules that Google publishes.
  • Count the actions that matter. You mark the action that makes your revenue as a key event, a contact request or a purchase. Reports then show how many times it happened, channel by channel.
  • Feed Google Ads. A key event can become a Google Ads conversion: your campaigns bid on it, and Google Ads and GA4 report the same counts (about key events).

How Does GA4 Work: Everything Is An Event

GA4 uses event-based data instead of session-based data, the big break with the previous version. Three parts make the measurement work.

  • The data stream. It is the pipe between your site or app and your property, and there are three types: Web, iOS and Android. A Web stream has a measurement ID that starts with G-, which the Google tag or Google Tag Manager install on your pages.
  • The event. Each interaction becomes a line: page_view for a page view, scroll for a scroll, generate_lead for a contact request. A three-page visit that ends with a form writes at least five lines in the logbook.
  • The key event. Any event can be marked as a key event, and that mark is what makes it count in the « Key events » column of the reports.

Events come through four doors, which the help page on events separates. Automatically collected events start as soon as you install GA4, such as the start of a session. Enhanced measurement is turned on in the interface, with no code: scrolls, outbound clicks, site search, videos, file downloads. Its scroll event fires the first time a person reaches 90% of the page height (enhanced measurement). Recommended events use names that Google defines, such as generate_lead or purchase. Custom events carry the name you choose. Google advises creating them only when no other type works, because most standard reports do not show them.

Four lines of a logbook covered with dots, the events; a reading sweeps across them and only two, marked as key events, get a ring and raise the counter.

A Real Example: 561 Key Events For 22 Requests

My own GA4 property became a case study. Until October 16, 2026, it had 14 key events: the contact request and the purchase, but also scrolls at 50, 75 and 100% of the page, time on page from 20 to 80 seconds, the view of the contact page in each language, and the start and the submission of a form.

From July 18 to October 15, 2026, GA4 therefore counted 561 key events for 22 real contact requests, plus one test. The organic social channel alone showed 277 key events for 76 sessions, and a single request. In the report, this channel looked like the best one on the site.

On October 16, the property went back to two key events: the contact request and the purchase. My recommendation is to keep two levels apart: the contact request and the purchase as key events, scrolls and time on page as plain events. Plain events help you understand a page; they never serve as a primary conversion in Google Ads.

The GA4 Channels, AI Assistant Included

GA4 puts each session into a channel of the default channel group, based on its source and medium. These rules are published in the help page on the default channel group, and you cannot edit them. One of them concerns every site: the « AI Assistant » channel groups visits from sources such as ChatGPT, Gemini, Deepseek, Copilot or Grok, while Google's AI Overviews and AI Mode stay in « Organic Search ».

On my site, this channel brought 5 of the 22 contact requests of the same period, all from ChatGPT. Direct brought 10, Google search 6, LinkedIn 1. The method to track these visits is in the article on LLM traffic in GA4, and the way GA4 reads a tagged link is in the guide What are UTM parameters.

GA4, Universal Analytics And GTM: Who Does What

Universal Analytics is the previous version, built on sessions. Its standard properties stopped processing data on July 1, 2023, and the Analytics help center announced the end of all access, interface and API included, from the week of July 1, 2024. Its data now only survives in exports made before that date, BigQuery included.

Google Tag Manager places the tags; GA4 receives and sorts what they send. The two work together: Tag Manager fires the GA4 tag at the right moment, and GA4 turns it into reports. The guide What is Google Tag Manager covers the first one.

Common Mistakes In GA4

  • Too many key events. This is the case of my site above: every scroll counts as a win, and the channel that scrolls the most looks like the best.
  • Data retention left at 2 months. On a standard property, the choice is between 2 and 14 months. This setting limits explorations and funnel reports, while standard reports keep showing their totals (data retention). In September 2026, out of the 36 properties I follow, 30 were at 2 months.
  • A custom event instead of a recommended one. A made-up name such as form_submit_contact works, and generate_lead is the one Google defines for a contact request, with a value recommended once it becomes a key event (recommended events).
  • Badly tagged links. A missing or misspelled medium sends the visit to « Unassigned », and the real channel loses a session it had earned.

The Limits Of GA4

  • Part of your numbers is estimated. GA4 includes cookieless measurement, and behavioral and key event modeling. The article on conversion modeling explains the mechanism, and the Consent Mode v2 simulator puts numbers on it for your refusal rate.
  • Caps on the free version. 30 key events per property, 14 months of retention at most, one million events per day in the daily export to BigQuery. Analytics 360 raises these limits (Google Analytics 360).
  • Google Ads data covers 36 months. In GA4, it lives in a rolling 36-month window, with a banner when your date range goes beyond it.
  • An app goes through Firebase. The iOS and Android streams of an app are measured with the Google Analytics for Firebase SDK, covered in the guide What is Firebase.

The Three Most Searched Questions

Is Google Analytics free? Yes, in its standard version: the official Google Analytics page presents it as free for businesses of every size. The paid version, Analytics 360, requires a contract and raises the limits: number of key events, retention period, volume exported to BigQuery.

What is the difference between Google Analytics and Google Search Console? Search Console measures what happens in Google before the click: queries, rankings, clicks. GA4 measures what happens after, on your site. You can link the two, and the integration adds organic search queries to GA4, limited to the last 16 months that Search Console keeps (connect Search Console).

What does « Direct » mean in GA4? It is the « Direct » channel. Based on its rules, GA4 puts a session there when the source is (direct) and the medium is (none) or (not set). In plain words, GA4 received no website of origin and no tag. On my site, this channel carried 10 of the 22 contact requests over the 90 days studied.

Open your property, go to « Admin », then « Data display » and « Key events », and count them. If there are more of them than actions that make your revenue, the cleanup starts there, before you touch your campaigns: once they are imported into Google Ads, they teach your bidding what to look for. This review is part of a Google Tag Manager and GA4 engagement.