Well...if all 4 leads qualified for the smart list, then Marketo will try to process them. The usual method would dedupe against the same email, but since AB takes a random sample, it's very likely it wouldn't dedupe at setup, it would dedupe at each email run.
Maybe Cheryl can confirm this.
The only way to handle this better is to clean up dupes in Marketo using the Possible Duplicates list or the dupe filter. I'd check with your SFDC folks on this.