Using GA with parameters reading from a excel file

I have a function that read initial conditions and some parameters from a excel file and calculates some outputs in a daily time step.

[Temp,DO, DOC, DOM] = solvemodel(m_start,m_stop,initfile,'sheet1',inputfile,'sheet1', parafile,'sheet1'); it mean that that the directory of three excel files (initfile, inputfile, parafile) should be given in order to run the model.

In parafile, there are 40 parameters..

Now I have the observation data of temp, and I would like to apply GA with the objective of minimizing the RMSE beteween observed and modelled temp. Among the 40 parameters of parafile, I just want to 4 of these parameters to e changed in the GA, others should be constant. How can I use GA toolbox or write the code to do so.