You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: dtd_pake_4d_data2fit.m
    path of file: ./repos/md-dmri/methods/dtd_pake
    
    the code of the file until where you have to start completion: function msf_fn = dtd_pake_4d_data2fit(s, mfs_fn, opt)
% function dtd_pake_4d_data2fit(s, mfs_fn, opt)

    
function msf_fn = dtd_pake_4d_data2fit(s, mfs_fn, opt)
% function dtd_pake_4d_data2fit(s, mfs_fn, opt)
%
% Error function fit
% Assumes powder averaging and axisymmetric b-tensors
% Eriksson et al., J. Chem. Phys. 142, 104201 (2015).
% http://dx.doi.org/10.1063/1.4913502

if (nargin < 3), opt = []; end