Are the few brown or black discharges at the beginning of the menstrual cycle (first 4 days) and at its end considered menstruation, which necessitates abstaining from prayer and fasting, knowing that the cycle sometimes lasts for 12 days and the period between cycles is irregular and close to 27 days?

The maximum duration of menstruation is fifteen days. So, if the bleeding does not exceed fifteen days, all of it is considered menstruation. Yellowish and brownish discharges are considered menstruation if they occur during the habitual period or are continuous with the blood. If you see yellowish or brownish discharge before the blood, it is not considered menstruation unless it occurs during your habitual period. Black blood is considered menstruation as long as it occurs within its possible timeframe. Brownish discharge at the end of menstruation, if it is continuous with the blood, is menstruation. However, if you see purity and then brownish discharge during the habitual period, it is menstruation; otherwise, it is not. In that case, purification and ablution for prayer are obligatory.