Try this
Clear[x, y, z, is, q, f];
q = x^2 + y^2 + z^2 - 5 is;
f[x_, y_, z_, is_] := Evaluate[q];
Now let's see what f is
?f
which shows you
Global`f
f[x_,y_,z_,is_]:=-5 is+x^2+y^2+z^2
so it appears that the contents of q have been used in the definition of f.
Try using f and see what happens
f[1, 1, 1, 1]
which gives you
-2
and that seems to be the result you are expecting.
So using Evaluate was the key.
