GA4 reporting data discrepancies: why are my conversions and user counts so off compared to UA?

Hey everyone,

Just pushed some updates to my SaaS app, trying to get a handle on GA4 metrics for a new feature launch. i'm hitting a wall though.

I'm noticing some pretty wild data discrepancies between what GA4 is showing and what I expected, especially around event counts and active users. It's making it super hard to trust the numbers for decision making. My old UA property, which is still running in parallel, shows very different figures for the same time period. for example, for a specific day, GA4 might report 80 conversions while UA shows 150 transactions, and user counts are similarly divergent.

// Example from GA4 DebugView for 'purchase' event firing correctly:
// event_name: 'purchase'
// user_id: 'user_123'
// transaction_id: 'TXN_456'
// value: 99.99
// currency: 'USD'

// But then, looking at the standard reports:
// Comparison for a specific day (e.g., yesterday):
// GA4:
//   Total Users: 1,200
//   Conversions (purchase): 80
// UA:
//   Users: 1,800
//   Transactions: 150

What are the common culprits for such significant gaps in GA4 data, particularly compared to UA? Are there specific configurations, filters, or event parameters I should double-check? i really need to iron out these data discrepancies to get reliable reporting. Also, any tips on debugging user counts?

Help a brother out please...