Statistical analysis for multiple dependent variables[biodiesel]?

Hi everyone!First post here :)
I am working on a biodiesel synthesis project via the acid route, and my objective is to see the dependence of 4 variables (x, y, z, w) on the yield of the reaction. For each variable a maximum and minimum limit was established, and I performed 16 experiments varying these limits.
I would like to find an equation like: yield = ax + by + cz + dw, could anyone tell me a way to accomplish this?
I have a macbook, and downloaded a SPSS v.24 software.
If they have any other software to indicate, I would be very grateful.
You could just run a linear regression model with this model:

yield = ax + by + cz + dw,

I believe that you need to declare the x, y, z, w as factors. Otherwise just code those with high values as +1 and those with minimum values as 0.