If you buy a 4S, buy a tach at the same time because trying to tune by ear isn't going to cut it! If you go the 2S route go with a 46 size and don't worry about it. The extra power will be very handy if the grass starts getting long. I have watched LT40s trying to get off the ground in long wet grass with 40LA engines on them, barely made it! The LT40s with the OS46 AX or FX engines din't have too much trouble.