When should a woman consider herself pure if the bleeding stops after four days, and then a brown thread or a yellowish or brownish spot appears?

A woman recognizes the cessation of menstruation by one of two signs: dryness (inserting a cotton swab and it coming out clean), or the white discharge (a white fluid that appears after menstruation). If she sees either of these, she has become pure, and ritual bathing (ghusl) becomes obligatory for her. As for yellowish and brownish discharges (al-ṣufrah wal-kudrah), if they occur during the period of menstruation, they are considered menses. However, if they appear after the menstrual period has ended and purity has been observed, they are not considered menses, and no attention should be paid to them, requiring only ablution (wudu') for prayer.