Pitchers with 4 strikeouts in one inning

The number of four-strikeout frames — made possible when a hitter takes first base after a third strike is dropped by the catcher plus either first is vacant or there are two outs — has grown exponentially in the 21st century. While there were no such achievements by a pitcher in nearly four decades from 1917-55, there were four in 2017 alone, and six the year before that.