[TexasSP]

IF you run two 4s in series, you are doubling the voltage over 1 4s. You need to run them parallel to achieve the same voltage. While you won't see any difference as far as the ESC goes, the packs would do better if you are talking 2 vs 1 pack of the same MAH. Running in parallel would double your MAH so if you are running 2 4000 MAH 20c packs versus 1, your amperage output goes from 80 to 160.