Ok gang, question ...

I am looking at this Futaba transmitter, "ATTACK 4YWD".  Its 4 channels.  From what I understand, a tank needs at min 5 channels in order to support forward, reverse, turning, shooting, elevation and turret.
This receiver is compatible with futaba receivers that are 5,6,7 and 8 channels.
What's the deal with a 4 channel transmitter being able to work with a 8 channel receiver?  What would be the point? 
If I were to plug in 5 servos into the receiver, but the transmitter was only 4 channels, then 1 of those items would be non-operational...correct?

I like Futaba from my rc car days, and overall it looks like a good model. 

Sorry if this is a silly q for some of you.  Still putting this stuff together.  Thanks
Paul

Older design Clark MFU's and older uses a 4 channel controller with different inputs to give a simulation of multiple channels.

Basic moves.
One stick two axis, forwards and back combined with left and right track movement.

Other stick for turret rotation and elevation. However when you push up past 75% on elevation you fire the cannon.

Additional examples of channel blending that the Clark uses.
Down and To Right on turret stick starts the engine.

Newer ones (that cost more) actually have more channels (8 I believe)

Futaba Attack at RCMart: $86.90US including receiver

http://www.rcmart.com/futaba-4ywd-24ghz-wr214gfe-p-37204.html?cPath=36_818

sorry i like my Turnigy 

https://hobbyking.com/en_us/turnigy-9x-9ch-transmitter-w-module-ia8-receiver-mode-2-afdhs-2a-system.html

In theory and mostly in practice a 4 channel radio will do everything you need to do to control a tank's movement plus basic functions. Tamiya always used a 4 channel (Attack) but it cheats a little with the trim. Clark uses different stick combinations on the basic 4 channel layout.

All the purpose built tank electronics (Elmod, Clark, Tamiya, IBU, Heng Long etc) maps the channels into different functions. Paul, what you have in mind is like basic flyer where every channel controls a servo only which does not have any purpose built electronics between the radio and the controls.

We like more than 4 channels so that we can control some basic functions with switches, but reality is that 4 channel is more than enough to run a RC tank.

Very insightful guys. Appreciate it! Been watching YouTube videos on this (yes fell into the YouTube black hole) and the flysky i10 looks very user friendly for a simple guy like me....