danzasmack is right. Four unknowns + three linear questions = infinitely many solutions.
Also, don't waste brains and ink and Excel on this, kids. There are
websites for stuff like this.
Pick anything you want for z, and the values of w, x, and y are:
w = -(2*z-1)/2
x = 2*z
y = -(4*z-1)/2
z = z
Since you said they all have to be greater than zero, pick any z so that 0 < z < .25