Can I do that: Plugging 4.8 V pack and one 6V pack on my receiver?
It doesn't make sense to connect 4 and 5 cell pack in parallel. If connected directly five cell pack would "charge" four cell one all the time making it totally useless for powering RX and servos and ultimately draining the 5 cell pack faster.
If connected through diodes it again doesn't make much sense because 4 cell pack would not be used until 5 cell one is thoroughly dry and diode would shave at least 0.3V off of it.
Your best bet is to use two identical packs in parallel, either directly or through diodes. If you decide to go with diodes use schottky barrier types with low forward voltage drop.