I'm working on a 4 layer PCB with internal 2 layers being power and GND... 2 outside layers are signal. Is it better to have a pour on the outside layers or just leave them empty except for the signals?
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.
Ground pours on layers other than the ground plane is generally not needed. One exception occurs if you are using SMT power devices (and even some thru-hole) the do their heatsinking via the pc board. In that case, having a nice big copper area directly under the device can be a very good thing.