[Sylvester]

Id say 5s because the motor you have (7xl) will run more efficiently with 5s. 4s will work too, but you will not have the same speeds and acceleration. On 4s it will also run longer and cooler. IMHO go for 5s. But if your pleased with the 12 cells your gonna be using for now, 4s would be a pretty big step, too!