Forcing number of bins on histogram

I've been constructing three histograms on data which is basically a response variable split by three treatment groups. I want the graphs all to have the same scale for the x and y axis so I've been using the basic syntax:

hist response, by(treatmentgrp)

I want each of the individual histograms on the plot to have 4 bins, so I've been specifying bin(4) after my by statement as such:

hist response, by(treatmentgrp) bin(4)

My problem is that only two of the three plots actually then show up with bin(4), the other has only 2 bins making the true shape of the response difficult to see.

Does anyone know how to force Stata to make all three histograms have 4 bins?