By now, pretty much everyone knows that Masahiro Tanaka has a flat four-seam fastball. Even Tanaka himself knows that, hence why he's stopped throwing it as much. Since his stateside debut with the Yankees in 2014, Tanaka has decreased his four-seamer usage every year. This year, he's reached an unprecedented level of four-seamer avoidance.
Brooks Baseball tells us that Tanaka has thrown his four-seamer just 14.3% of the time in 2017. It's a bold strategy, but one that makes sense. Tanaka's worst pitch is his four-seam, so he's throwing it less often.