well, a big reason is that the discharge ability of the battery is related to its size. say the battery can support 10c discharge, your 4000mah batteries can put out 40A each at that rate. if you put them together in parallel, now you can draw 80A. What I would suggest is, Invest in a whattmeter, then you can find out how many apms youre drawing. if its more than the discharge rate of 1 of the batts, then use both. if its under, well, you have an extra battery, now you get twice as much flying