Before us is chapter 6, mishnah 4. This mishnah continues to discuss the details of the Kohen Gadol's obligation to marry specifically a virgin.
"Kohen gadol lo yisa almanah":
The Kohen Gadol may not marry a widow. An ordinary kohen may marry a widow, and is forbidden only in a divorcee; the Kohen Gadol, however, is forbidden in both - in a divorcee and in a widow. This prohibition applies both to a widow from betrothal (where there was nothing but kiddushin) and to a widow from a completed marriage.
"V'lo yisa es habogeres":
The view of the first Tanna: A bogeres too is forbidden to him. A bogeres is a girl over whom six months have passed from the time she reached the signs of maturity. At that point the Sages said that she no longer has her virginity in full measure; rather, it steadily diminishes. And since it is stated regarding the Kohen Gadol "V'hu ishah bivsulehah yikach" - that he should marry a woman in a state in which she has her virginity - the first Tanna learns that she must have her virginity in full measure, and therefore he marries only a na'arah, that is, within the first six months after maturity.
The view of Rabbi Elazar and Rabbi Shimon: "Rabbi Elazar v'Rabbi Shimon machshirin b'vogeres" - a bogeres too is fit to marry a Kohen Gadol, for they interpret the word "besulah" as applying even to a partial measure of virginity.
"V'lo yisa es mukas eitz":
A mukas eitz is a woman who was never intimate with a man, but was injured in the place of her virginity by an object, such as a piece of wood, and she too has no virginity - even though this did not come about through intimacy. Her too the Kohen Gadol may not marry.
An ordinary kohen who betrothed a widow and was appointed Kohen Gadol:
An ordinary kohen who betrothed a widow - this is permitted, since he is not a Kohen Gadol. And if he was appointed to be Kohen Gadol before he completed the marriage with her, the mishnah says: "Yichnos" - he is permitted to bring her into his home, since she is already considered his wife.
"Uma'aseh b'Yehoshua ben Gamla":
Yehoshua ben Gamla betrothed Marta bas Baisus, a wealthy widow, and the king appointed him to be Kohen Gadol (and as the Gemara explains, the appointment came about with his wife's help, out of her own pocket). So we have a case where he was appointed Kohen Gadol while she was still a widow and while there was nothing between them but kiddushin - "uk'nasah" - and the Sages permitted him to do so. And even though the manner of the appointment itself was not praiseworthy, proof can still be brought from here that he was permitted to bring her into his home even though she is a widow, because he had already performed kiddushin with her.
A shomeres yavam who came before a Kohen Gadol:
A woman whose husband died without children waits for yibum and comes before her yavam, her husband's brother. If the yavam is a Kohen Gadol, or was appointed Kohen Gadol before he performed yibum, he has only a zikah with her - a bond of yibum, a potential bond, but this is not kiddushin.
And although had he performed kiddushin with her she would be considered his wife and he would bring her into his home, here, by the very fact that his brother died and she came before him for yibum, she is not his wife - and therefore he may not bring her into his home, for this would be like marrying a widow.
One who was already a Kohen Gadol and whose brother died:
He is obligated to perform chalitzah for her, since this prohibition does not carry kareis. Therefore he must deal with the zikah and remove it by means of chalitzah, but he does not perform yibum with her, because she is a widow.
In summary: In this mishnah we learned the details of the prohibitions that apply to a Kohen Gadol - a widow (whether from betrothal or from a completed marriage), a bogeres (in the dispute between the first Tanna and Rabbi Elazar and Rabbi Shimon), and a mukas eitz, all derived from the verse "V'hu ishah bivsulehah yikach". We also learned that an ordinary kohen who betrothed a widow and was then appointed Kohen Gadol may bring her into his home, as in the case of Yehoshua ben Gamla and Marta bas Baisus, whereas a shomeres yavam, with whom he has only a zikah, may not be brought into his home; rather, he performs chalitzah for her.