$u(x) = C_1 |x|^{-2} + C_2$ for harmonic $u \in C^2 (\mathbb{R}^4 \setminus \{ 0 \}) , u \geq 0 $

Question

Problem

Assume $u \in C^2 (\mathbb{R}^4 \setminus \{ 0 \}) $, and $ u \geq 0 $ is harmonic in $ \mathbb{R}^4 \setminus \{ 0 \} $.

Prove that $u$ has the form that $u(x) = C_1 |x|^{-2} + C_2$ for $x \in \mathbb{R}^4 \setminus \{ 0 \}, $ where $C_1, C_2 \geq 0$ are constants.

My PDE textbook is Walter A. Strauss's Partial Differential Equations: An Introduction. I have no idea how to deal with this problem, what I know is the maximum principle for harmonic function. Is there any properties or theorems related to positive harmonic function?