My menstrual cycle has changed. Before, it used to last for 5 o 6 days. Now, in the 4th day the bleeding stops and I start bleeding again in the 5th day. When should I do my ghusl? Shall I do it in day 4 or wait till everything is finished (thus ignoring that the bleeding had stopped on day 4). May Allah reward you with goodness.
You wait until you see the Tuhr sign, the purification sign. This sign is either a white liquid, or a complete dryness. A dryness can occur in the middle of the menstrual period, so you consider this day also from the menstrual period, and you make ghusl after your menses is finished (i.e. After tuhr sign). This is the fatwa of Shaikh Ibn Al-Uthaymin.