Some futaba servos recomend 4.8v 4cell packs, not all. This is because there are some servos that cant run 5cell packs because the voltage is too high. A freshly charged 5cell pack will be well over 6volts and some servos just dont work correcly over 6volts. even JR has some troble with this its not only futaba.

as far as the NIMH Vs NiCad...

Nicads wont be as much over their rated capacity unlike NIMH's a freshly cahrged Nicad might be a little above 6 volts, but the Nimh will be well above 6volts

Most Nicads have a higher discharge rate than NiMH's. If runing multiple high load servos some Nimh migh over load