2,089. Patterns of Four and Five Days
Hilchos Issurei Biah 7:17
If a woman’s pattern is to see blood for four impure days followed by four pure days, she may only engage in intimacy on the day after her niddah days. After that, she may never engage in intimacy again.
Hilchos Issurei Biah 7:18
If her pattern is to find blood for five impure days followed by five pure days, she may engage in intimacy on three days after her niddah days, after which she may never engage in intimacy again.