This is likely because the email found by waterfall provider #4 failed validation.
When an email waterfall runs, it continues to the next provider if the current one finds an email that doesn't pass validation. The email might have been found but marked as invalid by ZeroBounce (our default validator).
To investigate, you can unhide the validation columns to see exactly what happened:
1. Open the Columns Panel
2. Unhide the validation columns
3. Check if the email was found but failed validation
This is a common scenario - the waterfall continues running until it either finds a valid email or exhausts all providers.