You have to distinguish between 4-channel-helis (koax and fixed pith (where koax may use even less channels)) and 6-ch-helis (collective pitch).
The second needs the same four channels as the 4-channels does need: throttle, rudder, aileron and elevator. But because the six-ch-helis do not control the height of flying only with the motor speed but with the angle of the blades (blowing more or less air down and up) so for this purpose they need another channel "pitch" (which moved all three servos up and down together to change the blade angle). The sixth channel is mostly used for the rudder gyro control.
So reducing the number of channels does not turn a six-channel-collective-pitch-heli into a 4-ch FP - never. The only way to do this is to let the receiver calculate the additional pitch channel values like it is done when you have an E-Flite Blade mCP-X with a simple 4-channel-transmitter (I had this with my RTF Blade Nano CP-x).
