Bourbon is one of the most heavily regulated spirits categories in the United States. But once distillers comply with the basic requirements – at least 51% corn, new charred oak barrels, no colorings or added flavorings – the room for experimentation is tantalizing. One of the sub-categories of bourbon that offers the most room for innovation is a little-known style called four-grain bourbon.

There are just a handful of true four-grain bourbons on the market. Like all bourbons, four-grain bourbon must be at least 51% corn. And like standard bourbon, it also contains some percentage of malted barley to…