Yevamos, Chapter 4, Mishnah 11. The Mishnah deals with a case of brothers married to four women, where the women are not related to one another.
"Arba'ah achin nesu'in le'arba nashim" - four brothers married to four women:
The Gemara notes the precise wording: the intent is not to a family of exactly four brothers, but rather "of brothers" - a family with many brothers, four of whom, married to four women, died, while other brothers remained alive.
"Ve'im ratzah hagadol shebahem leyabem es kulan - harshus beyado" - if the eldest of the surviving brothers wishes to perform yibum with all of them, he is permitted to do so. Even though there are four separate obligations of yibum here, he may fulfill them all. The Gemara cautions, however, that he must be able to support them financially.
"Mi shehayah nasui lishtei nashim umes" - one who was married to two women and died:
"Bi'asah o chalitzasah shel achas mehen poteres tzarasah" - the yibum or chalitzah performed with one of them exempts her co-wife. This is a principle we have already seen: yibum or chalitzah with one of the two widows is enough to exempt her co-wife. It is enough to perform yibum with one alone, or chalitzah with one alone, and thereby the obligation of yibum is removed and the zikah is lifted from both women at once.
"Haysah achas kesheirah ve'achas pesulah" - if one was fit and one was unfit:
Here the Mishnah offers practical advice. Suppose that of the two widows, one is fit for kehunah - permitted to marry a kohen - and one is disqualified from kehunah, such as a divorcee:
"Im hayah choletz - choletz lapesulah" - if he is going to perform chalitzah, he should perform it with the one who is already disqualified from kehunah, since chalitzah itself disqualifies a woman from marrying a kohen.
"Ve'im hayah meyabem - meyabem lakesheirah" - there is nothing preventing him from performing yibum specifically with the one who is fit for kehunah, since the process of yibum creates no disqualification whatsoever.
In summary: in this Mishnah we learned that the eldest brother may perform yibum with all four of his brothers' widows, that yibum or chalitzah with one of the deceased's two wives exempts her co-wife, and also the practical guidance: chalitzah should be performed with the woman who is already disqualified from kehunah, since chalitzah disqualifies, while yibum should be performed specifically with the one who is fit, since yibum does not disqualify.