The 16-35mm f/4 ZA was the third zoom released for the Sony full-frame E-mount system in 2014 and completed the f/4 constant aperture series. At the time, the trio was the only option available if you wanted quality variable focal length lenses.
Four years later, Sony users have much more choice including a set of faster f/2.8 constant zooms. The latest to be realised was the 16-35mm which is part of the high quality G Master series.