If you're refering to the number of channels on a receiver in order to fly electric 3D then - yes - 4 is sufficient. You will have to use a Y-harness on the ailerons (as mentioned above) if running 1 servo in each aileron. The typical "negative" shift (as in Futaba) set up is:
Ch.1 - ailerons
Ch. 2 - elevator
Ch. 3 - throttle <<<this is where the ESC should plug in to
Ch. 4 - rudder

If running glow then you will need at least a 5 channel receiver (all the above plus 1 to plug your battery in to).

I would also suggest that if you're flying in a city park or in a heavily populated area that you also get a dual convesion receiver as well. I've used a single conversion in the past and had nothing but problems with hits - to the point of putting my plane in. I've never had any interference with the dual conversion although it is still possible but it lessens your chances of getting hits.

If you're wanting to include flaperons or spoilerons then you will definately need at least a 6 or 7 channel Rx. depending on your set up. Although these are not necessary to do 3D. Some may use them but most folks that I know don't. <<<for whatever this is worth.

The radio is another important factor (besides your plane and gear) in 3D. You will need a programmable one - for setting up dual rates, expo, servo travel and any coupling in the control surfaces that you may want. Your typical 4 channel radio doesn't provide these capabilities however you would still be able to do things like hover and knife edge with no problem depending on your skill and your plane of course. You just will not get the increased throws that you need to do a lot of the extreme stuff nor will you get the extra help that programming provides.

Good luck!