Most ecommerce customers use multiple devices before making a purchase. They might browse your Shopify store on their phone, compare products on their laptop, and finally buy on their tablet. Without cross-device tracking, GA4 counts these as three separate users. With it, GA4 can recognise them as one person and provide a much more accurate picture of the true customer journey.
The Three Identity Methods in GA4
GA4 offers three methods for tracking users across sessions and devices:
1. Device ID (Default)
The default method. GA4 assigns a random identifier to the browser or app instance. Works within a single device and browser, but cannot follow users across devices or when they clear cookies.
Limitation: if a user visits your store on their phone Safari and then on their laptop Chrome, GA4 sees two different users.
2. User ID
When a user is logged in to an account on your Shopify store, you can send GA4 a unique identifier for that user (their Shopify customer ID, hashed). GA4 uses this identifier to connect sessions across different devices for the same logged-in user.
How it works: you set the User ID in your GA4 configuration code: gtag('config', 'G-XXXXXXXX', { 'user_id': 'HASHED_CUSTOMER_ID' });. Do not send the raw customer ID — hash it first. In GTM, you can read the customer ID from the Shopify dataLayer (dl_user_data event) and pass it to the GA4 tag.
Limitation: only works for sessions where the user is logged in. Guest checkout sessions or pre-login browsing cannot be linked.
3. Google Signals
Google Signals uses Google Account data. When a user who is signed in to their Google account (Gmail, YouTube, Google Maps, etc.) visits your website with Ads Personalisation enabled, Google can recognise them across devices using their Google account identity.
How it works: you enable Google Signals in GA4. No code changes are needed. GA4 automatically starts using Google’s cross-device graph for users who are signed into Google accounts.
Limitation: only works for users signed into Google accounts with personalisation enabled. Does not cover Apple ecosystem users or Google account holders who have disabled personalisation.
How to Enable Google Signals in GA4
In GA4, go to Admin → Data Settings → Data Collection → Google Signals Data Collection. Click Get Started and follow the setup prompts. Enabling Google Signals requires confirming that your privacy policy covers the data collection involved.
Once enabled, GA4 will begin using Google account data to improve cross-device attribution. This may cause the number of reported “users” to drop slightly as cross-device sessions that were previously counted as multiple users are now correctly attributed to one.
How to Enable User ID in GA4
User ID implementation requires code changes to your Shopify theme or GTM setup:
- Generate a unique identifier for each Shopify customer (use the Shopify customer ID or a hashed version)
- When a user is logged in, make this ID available in the page’s JavaScript (Shopify’s Liquid can inject it: {{ customer.id }})
- In GTM, create a JavaScript variable that reads the customer ID from the page
- Pass the customer ID to GA4 via the user_id parameter in your GA4 configuration tag
If you are using the Shopify dataLayer (dl_user_data event), the customer email is often available there. Hash the email with SHA256 before using it as the user_id.
Blended Identity: Combining All Three Methods
GA4 uses a blended approach: it prioritises User ID when available, falls back to Google Signals for Google account holders without User ID, and uses Device ID for everyone else. This layered approach gives you the best cross-device coverage without requiring any single method to work for all users.
Privacy Considerations
Google Signals requires informing users about data collection in your privacy policy. If you serve EU visitors, you need explicit consent for Google Signals (GDPR compliance), and Google Signals should only be active for users who have consented. Implement this via Google Consent Mode v2 with consent signals linked to your cookie consent tool.
Get Your GA4 Cross-Device Tracking Set Up
Cross-device tracking improves the accuracy of your user counts and customer journey analysis. If your Shopify store has a significant registered customer base, User ID implementation gives you meaningful cross-session visibility for logged-in shoppers.
Book your free Shopify tracking audit here and we will review your GA4 identity configuration and confirm cross-device tracking is set up correctly.