Hi
The frequency (MHz) should match. If you buy a 6ch transmitter, you can 4- and 6 channel receivers, just as long as they both are on 72 MHz. If you buy a 6 channel receiver and a 4 channel transmitter, you will (of course) never be able to use the 2 extra channels on the receiver.
So, when buying a transmitter, think of the future. If you will be flying airplanes in the future, I suggest the 6 channel tx+rx
Regards,
Jan