Well the 4 channel unit is out of the question simply because you would not have gain for the FMA and on/off for autopilot.
The 6 channel unit from hobbycity looks like it has channel 5 and 6 proportional with knobs, so this would work fine.
One of the knobs for the FMA and the other for autopilot on/off (failsafe).
I personally fly with 8 channels for flight and failsafe/fma, plus extra radio for pan/tilt and payload trigger... But it all depends a lot on what kind of airframe you will be using. If your airframe has for example retracts, or flaps, or segmented ailerons 6 channels will not be enough.