Goal of the fourth month is the same as the previous months, paint a warband. Again, the warband can be anything you like, as long as it is a warband rules-wise. Lone hero is a warband, hero with warriors is a warband, but a lone warrior is not a warband.
And then the bonus goal. In addition of a warband, each participant can paint as much as they can! Anything counts, so this should be rather easy.