Optimal Capacitor Placement on DC/DC Convertor
Option 2 is the most suitable given the choices presented.

Why:  Ground planes are always better for re-circulating currents than tracks. (eliminating options 3 and 4).  The capacitors are likely to serve dual purposes: first for recirculating currents, second bulk capacitance of the output rail.  Given the later that eliminates option 1.  Hence option 2 is the most suitable given the choices.

However, you need to consider input ripple current and how to filter that, so ideally your should have a series inductor followed by your bulk capacitance with low esr before the input supply.  This will keep input ripple localised.  

Next splitting ground planes is never a good thing.  You are better off having an isolated ground copper shape around your DC/DC converter with all of your filter components (input inductor and capacitor and output capacitors) attached to the ground shape and then connect it to your overall PCB ground plane.  Ideally you should have a ground plane on an internal layer and isolate it from your through hole component with a ground shape around the DC/DC with filter components, and then stitch your local ground shape to the plan around the edge or in one location.  

this will achieve two things: one you have a continuous ground plane across your whole board, two your recirculating currents are kept on the local ground shape.