sigma/(4*n) is an approximation. I think we should use the full correction. E.g.:

% std of a sample underestimates sigma (see http://nbviewer.ipython.org/4287207/)
% This can be very big for small n (e.g., 20% for n=2)
% We can compute the underestimation bias:
stdev = median(std(dataB0,0,1));
bias = stdev * (1 - sqrt(2 / (n-1)) * (gamma(n / 2) / gamma((n-1) / 2)));
% and correct for it:
sigma = stdev + bias;