In February 2015, Hewlett Packard announced a nozzle upgrade for their T-350 production inkjet presses. (HP HDNA - High Definition Nozzle Architecture).
According to industry consultant David Zwang, this upgrade "brings with it support for 5 levels of grey per pixel" (David Zwang article here).
A comparison of the quality increase with Multi-Phase, using the article's thumbnail of the B&W photo image can be seen here.
If one excludes the extremes of 0% and 100%, 5 graylevels represents a 50% increase in the number of middle output tones, from two (0-33-67-100) to three (0-25-50-75-100).
That comparison, between 4-graylevel and 5-graylevel Multi-Phase Screening, is shown below for the ISO Musicians image at 1200 dpi. For example, the shadings on the swan wings are
noticeably smoother with five gray levesl. There is a definite benefit to an incremental increase in graylevels. The largest quality increase appears to be from bi-level to 3
graylevels, with diminishing but definitive improvements from there.
MPS 4 level vs MPS 5 level (click to zoom in)