Not all batteries are the same. 2/3 A NiMh cells - typically found in RX packs - have a '1 C' charge rating. Or, they are rated to be safely charged at 1 amp per 1000mah rating of the cell. So, if there were a 2000mah rx pack it could safely be charged at 2 amps. Charging it at 4 amps could kill a cell. Sub C cells (stick pack) however, can usually be charged at '2C'. For instance a 4000mah NiMh stick pack could be charged at 8 amps.

If you are charging you rx pack at 4 amps, you can expect performance to decrease.