Layout of 4 layer PCB - pour?
Back in the days, pouring all layers was done in order to minimize the amount of copper etched during the PCB manufacturing process. The more homogeneous amount of copper across the PCB the better is the etching process.

Nowadays there is no point in pouring with copper all layers 

Pouring outer layer doesn't improve the shielding and the impedance necessarily. These are critical points and they must be considered in a proper way.

In general, you don't need to pour outer layers.