Resolves YES if either the American League Championship Series, the National League Championship series, or the World Series is finished in 4 games. Will resolve YES early if either of the LCS finishes in four games.
Background: The two pennant series (LCS) and the World Series are best-of-seven, where the series concludes when one of the two teams competing wins four games. When one team wins four games in a row, it is called a "sweep."
I will follow in a comment where I've been mathing about the total number of games played during the MLB postseason, which is related to this market.
When I wrote out the possibilities of combinations (e.g. ALCS 4 games, NLCS 7 games, WS 6 games), I counted 38 possibilities where one of the three remaining series contained at least one 4-game result. That's a 59.25% likelihood, but I figured it's close enough to a toss-up to make a market for it.