Consider specifying bin edges rather than the number of bins. If you want four bins centered at 0, 90, 180, and 270, then you need to specify five edges. In the example below, the edges are 0 90 180 270 360, but offset by 45 degrees. This gives you four bins centered at 0, 90, 180, and 270 degrees. Is that what you want?
% test data
y = 2*pi * rand(1,100);
edges = (0:90:360) + 45;
edges = deg2rad(edges);
polarhistogram(y, edges);
