Difference between the four buttons on bottom vertical face of Braillant [on hold]

After watching the YouTube video on how the Humanware Brailliant keyboard works, I am still perplexed about why there are four buttons at the bottom rather than two. I was told two of these advance the entire Braille forwards our backwards.

What do the other two buttons work?

How can I advance the Braille partially indeed of annexing by a whole full keyboard-length number of braille keys?

Having asked that, and suspecting that the four bottom face keys scroll a screenful or half a screenful back and forth, respectively, how would you improve this design to make the scrolling possible by any number of braille keys (perhaps by using two of these four buttons in combination with the cursor router keys above the Braille display)?

Thanks.

It some to me the four keys are for panning the display back or forward, or moving up and down through a set of lines.