I don't really understand what is it. Could someone enlighten me?

As you hold the shift key or the middle mouse button down, the speed at which the view moves around is 4 times greater. If you hold both the keys down at the same time, it's 16 times greater.

Pretty handy for larger maps.

is it possible to have 4x mouse speed always on except when someone emulates 1.5 speed?

i find the extra responiveness to feel better.

is it possible to have 4x mouse speed always on
Plug a toothpick or a small coin between your keys.

Or just turn up your Windows' mouse sensitivity, hehe.
Maybe there'll be a scroll sensitivity setting in the future.