Be careful using Ni-MH's in your transmitter..Nickel-based cells only have 1.2 volts per cell, as opposed to 1.5 for alkalines. The voltage in your radio will drop when you use Ni-MH's, regardless of how well they are charged.
For the same reason, you must run 5 cells in your receiver pack to get 6.0 volts like you do from 4 alkalines; that's why receiver packs are 5 2/3 A cells instead of 4 AA's.