4
It's simple geometry:
A planet absorbs energy from its star with its geometric cross-section $\pi r^2$.
The same planet re-radiates the energy via its complete surface $4\pi r^2$.
In equilibrium, in the incoming energy equals the re-radiated energy:
$$
E_{in} = E_{out}\\
\pi r^2 \cdot S \cdot (1-A) = 4\pi r^2 \sigma T^4
$$
Divide by the surface area of the ...
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