That's from the help file:
    Trade 4’s: There is a type of soloing called "Fours" soloing - Band-in-a-Box can generate the first four or the second four bars. Taking 'Fours' refers to a solo lasting four bars, usually followed by a different melodic solo (or drum solo) for the next four bars etc. It is fun to solo along with Band-in-a-Box in 4s mode, letting the computer solo for four bars, followed by your solo.

The modus allows for Trade 2's and Trade 8's too. In addition, check out the BB add-on CopyMe by Miles Black.

Martin