How do you declare an A/b email test winner before the 4 hour mark?
By default the system requires 4 hours to automatically send the winner.
Here is where the docs mention the 4 hour rule:
Schedule the A/B Test - Marketo Docs - Product Documentation
You could potentially manually select the winner if the 4 hour wait is too much:
Define the A/B Test Winner Criteria - Marketo Docs - Product Documentation
According to Devraj Grewal there is a workaround "...what you can do is run another email program and in the smart list exclude those who already were sent the email."
Check out this thread Ab testing