Given a function f[x_,y_,z_,a_]:=x^2 + xy+ yz + z^3+a^2 (just for example), I want to 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 have no idea how to proceed.