1)
Why did they go specifically four times a year?
1.
Rashi: They established this for a statute.
2)
What is the meaning of "Lesanos"?
1.
Rashi, Radak citing Targum Yonasan: It is to lament (her death - Malbim, according to Chazal).
2.
Radak, Malbim: It is to talk with her and console her. The rest of the year she was isolated.