how to megre vectors database in a unique one, and then get the best fitting

I have Four variables (T1, T2, T3 and T4)-in y-axes-I want to merge them without average- into One database, lets call it T_total. Then I want get a best fitting for T_total. The fitting should be as a power function B^c. I attached my my image.Fig includes Four vectors (Black (T1), Blue(T2), Green(T3) and Red(T4)).