Surface Integrals, Stokes & Divergence · Hard problem

A pollutant source has outward flux density field $\mathbf F=(x^3,y^3,z^3)$ in model units. A monitoring buoy totals the outward flux of $\mathbf F$ through a sphere of radius $R$ centered at the source. How many times larger is the total flux through the radius-$4$ sphere than through the radius-$2$ sphere?