What should a girl do who menstruates every 28 days, and after performing ghusl from her previous period, light red, pinkish discharges come down for two days, followed by clear discharges, then continuous yellowish discharges until the fourth day?

What you saw is not menstruation (hayd) because thirteen days (minimum purity period) did not pass between it and the previous blood. Nor can it be considered a single menstrual period with the previous one, as their combined duration exceeds fifteen days (maximum duration of hayd). You are experiencing istihadah (non-menstrual vaginal bleeding), and you must review what is required for a woman in a state of istihadah.