4 mishnayot! Practical details about kohanim and whom they can marry. For example, a kohen who is betrothed a widow, and is later appointed to be the kohen gadol, he's fine, they can marry. Plus, a story to make the point: R. Yehoshua ben Gamla. Also, how the kohen gadol cannot do yibum, only chalitzah. Also, the case of the aiylonit and the kohen gadol and the mitzvah of p'ru ur'vu. Plus, the kohen gadol is actually not supposed to marry too young of a girl. Also, a man may not disregard the mitzvah of being fruitful and multiplying when he gets married - unless he already has children. The details of which, are of course, disputed by Beit Hillel and Beit Shammai.