One way to do this is to use the ruler object representing the Y axis and change the TickLabelFormat for that axis. Using a slightly modified version of one of the examples from the histogram documentation page, let's create a histogram.
x = randn(10000,1);
h = histogram(x, 'Normalization', 'pdf');
Get the handle to the axes in which the histogram is located.
ax = ancestor(h, 'axes');
Get the handle to the Y axis ruler of that axes.
Y = ax.YAxis;
Change the TickLabelFormat to something that provides 4 decimal places.
Y.TickLabelFormat = '%0.4f';
