How to find values for four variables that yield a minimum output?

I have a worksheet that calculates an output parameter as a function of four input variables - i.e of the form y=f(a,b.c.d). I need to find the values of a, b, c, d that yield a minimum value for y. Currently, the best I can do is alter a,b,c,d using stepper format until I can manually find an approximate minimum.

I do not have a mathematical formula for y=f(a,b,c,d), so cannot use calculus etc to find the minimum.

Can anybody suggest how I can do this?