The brain is incredibly complicated and there is a lot of noise. More channels do make a huge difference. It's not like each channel is associated to one direction or something and you only need 4. Also, to add to the other users point, the signal strength is much stronger inside than out since the skull acts like a barrier.

Your example is the equivalent of saying you don't need a keyboard because you can use a computer with just the arrow keys and the spacebar.

Although BCIs are already a thing, the difference between what you can do with external compared to internal ones is probably quite vaste at our current tech level and will probably lead to better external ones at the same time.

You also don't control the electrodes, you just receive the signal from them and they are obviously all being sampled.

Well you'll need three states which are easily translated to the commands.

The more channels you have, the easier it is tor recognise any specific configuration of brainwaves.

It's not like thoughts are as easy to pinpoint as spatial coordinates. You don't really decide what your specific brainwave output will be when you focus on thinking "up". It will be a combination of different brain waves, and the more accurately you can measure that, the easier it is for the computer to pick up when you're thinking "up".

Try playing a tune with just three tones available to you, it'd be hard.

The technology isn't new, but the resolution sort of is.

Hell they used the tech in a House episode in like, 2009.

What do you mean "if you can't control them"? The channels are how many bands you're reading, and they are using quite fancy computers to do the reading. What's there to control? BCI is just a fancier interface than a keyboard, it's still only one-way, so I don't understand what you mean by "control" in this context.