Let \( S \) be the unit sphere in \( \mathbb{R}^n \), centered at the origin, and \( P_1 P_2 \dots P_{n+1} \) a regular simplex inscribed in \( S \). Prove that for a point \( P \) inside \( S \),
\[
\sum_{i=1}^{n+1} (PP_i)^4
\]
depends only on the distance \( OP \) (and \(n\)).