Adding outlet in middle-of-run
You can wire an single additional Romex from the 2nd electrical box off to a 4th for outlets. Just make sure that you use the proper gauge wire (should be 12 AWG for 20A circuit). 

If the current 2nd outlet is wired in a manner where the incoming wires are attached to the one set of screws and the outgoing run is connected to the other set of screws on the outlet then you will have to do a little juggling around to accomodate the added outgoing run. I would suggest to remove all current connections off that outlet and use good quality wire nuts of the correct size to join all the live wires (blacks), three from the Romex cables plus a 4th pigtail together. Do the same for all the neutrals (whites). The pigtails would connect to the 2nd outlet to connect it to the circuit. Lastly all the safety grounds (bare or greens) need to be wired together, connected to the box and to the green screw of the outlet.