Open Google Cloud Console → IAM & Admin → Service Accounts. Create a service account (any role, the role doesn't matter for GA4).

Click the service account → Keys → Add Key → Create new key → choose JSON. Download the file.

Open the downloaded JSON file and copy:

client_email → paste into the Service Account Email field above.
private_key (the whole PEM block including -----BEGIN PRIVATE KEY----- and -----END PRIVATE KEY-----) → paste into the Service Account Private Key field.

Open Google Analytics → analytics.google.com → Admin (gear icon at the bottom-left) → under your property, click Property access management.

Click + Add users, paste the same client_email, role Viewer (or higher), and save.

Your GA4 numeric Property ID is shown in Admin → Property → Property details (top-right). Combine multiple with the format id:name,id:name (the name is only for display).

Contact the Google Analytics Viewer team

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GA4 Configuration