RE: 2 Rx batteries ?
I believe the only servo's that Futaba has listed at 4.8volt only are high end digitals using nylon gears or special purpose, i.e. gyro. The F3A Futaba digitals I use are 4.8v only. But unless you plane on putting $100 digital F3A/Pattern servos in your 40 stick, don't worry about it. I'm not aware of any Futaba RX that is rated for 4.8V only. Use 5 cells if you want too. Be aware, you will not get as many flights from a 5 cell pack of the same size as a 4 cell pack.