Given a function `f[x_,y_,z_,a_]:=x^2 + xy+ yz + z^3+a^2` (just for example), how can I randomly choose values of `x,y,z,a` (with constraints that all $x,y,z$, and $a$ are positive, and `Sqrt[x^2+y^2+z^2]<=1`) and plot `f[x,y,z,a]` against `Sqrt[x^2+y^2+z^2]` for say 100 points $(x,y,z,a)$?

I am sure this is doable in Mathematica, but I have no idea how to proceed.