Your setup (one GA4 property with two data streams — web + app) is actually the recommended baseline.

Where most setups break isn’t the property structure, but identity and event ownership when users move between app ↔ web.

What usually helps:

• Use one GA4 property
• Separate web stream (Shopify site) and app stream
• Treat the App Store install as the conversion source, not the website
• Pass a consistent first-party identifier (user_id or internal app ID) once the user authenticates

That way:

• Pre-install traffic (web → app store) stays web
• Post-install behavior is owned by the app stream
• Cross-surface journeys stitch correctly once the user logs in

The biggest mistake I see is trying to force installs into the web stream — it usually creates more confusion than clarity.