That the factor of 4 pi appears in force equations (such as Coulomb's law or Gauss's law) as a geometric consequence of spherical symmetry (representing the surface area of a unit sphere, 4 pi r^2) is a fundamental geometric and physical fact that requires no citation.
The claim is a straightforward mathematical and physical fact (the appearance of 4 pi in inverse-square laws due to the integration over a sphere's solid angle of 4 pi steradians). None of the retrieved papers specifically target or test this exact pedagogical derivation as their main finding, making the matter common knowledge within physics and geometry.