Create a general fit_4_models function based on the one at pbi

[surh]

check functions at:

https://github.com/surh/pbi/blob/8da7b8961cfeef15e2870a3219adf368c29da2ef/functions_prrpropep.r#L204

and

https://github.com/surh/pbi/blob/8da7b8961cfeef15e2870a3219adf368c29da2ef/functions.r#L125