i dont think flying for 20 minutes on a single charge is a good idea or if possible. things will get hot. as far as going with the 4-cell setup for longer flight times....its defintely a good way to get longer flight times. make sure you have a motor and esc that's capable of doing 4S. adding a bec will also prove benefitcal. you will have to lower your pinion. and lastly but just as important is the bird capable of the extra weight added? adding weights will charge characteristic of the bird.

most trex owners just use the 3S setup as that what the 450 is designed for. just get more 3S lipos.