Schematic plot of the 4s radial distribution function 4πr2ψ4s2. Blue represents regions within which the wave function is negative and red represents regions where the wave function is positive.
For s-orbitals, the radial distribution function is given by multiplying the electron density by 4πr2. By definition, it is independent of direction.
In the case of s-orbitals, there are a number of radial nodes that separate the largest, outer, component from the inner components. The number of nodes is related to the principal quantum number, n. The ns orbital has (n-1) radial nodes, so the 4s-orbital has (4-1) = 3 nodes, as shown in the above plot.