I dont think you can use more than 3 cells with your runner 250 as the ESC’s are only rated for 11.1V, but I could be wrong.
However if you are looking for more flight time you can try upgrade to a 3000mAh battery, but I am not too sure about the maximum takeoff weight for the Runner 250, so if you use a battery that is too heavy, even if it is bigger you could get lower flight times as the motors need to work harder to lift your quad. We have a guide on how to choose the best battery for your drone.
But otherwise if you do want to use as lightly bigger battery you can always solder a matching connector for it to work onto your Runner 250.