What is the ruling on performing ghusl and prayer after the cessation of light bleeding for hours on the fourth and fifth days of the menstrual cycle, then seeing light bleeding again after ghusl, and is it permissible to delay ghusl until the fifth day to be certain of purity?

The relied-upon fatwa (definition) is that if a woman sees purity, she must perform ghusl (definition) and pray. If the blood returns during a time suitable for menstruation, she reverts to being a menstruating woman. Then, she performs ghusl again once it stops. What the questioner is doing is correct. There is another opinion by Ibn Qudamah that permits waiting for a day and a half after seeing purity during the accustomed period. This opinion can be followed by someone whose knowledge and religiosity are trusted, but the first opinion is more cautious.