Hi Bernhard,
We are checking with Futaba to get their take on it. Generally speaking, when you have a 4 cell battery which is a 4.8 nominal battery, you will typically have a voltage reading of up to 5.8 V after charging. This will drop down of course when it gets discharged, but still you start out at almost 6 V.

I will let you know what we get to know from the servo manufacturers.

Gerhard