All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If your normal period lasts for four days – as understood from your saying "My menses stops on the 4th day" – and you became pure (by finding one of two signs: absolute dryness of the vagina from blood or seeing the white discharge), then the brownish or yellowish discharge you may see after getting pure has no effect. This is not considered menstrual blood. Umm ‘Atiyyah was authentically reported to have said: "We did not count the brownish or yellowish discharge as menses after getting pure." [Al-Bukhari and Abu Daawood, and this wording is Abu Daawood’s] Hence, you are not required to make Ghusl in this case; you only have to clean the genital area.
However, if your normal period is seven days and the blood stops on the fourth day, and brownish discharge comes out on the 5th through the 7th day, this brownish discharge is considered menses. Muslim jurists stated that brownish and yellowish discharge during the woman's normal period is considered menses. Therefore, do not perform prayer on the 5th, 6th and 7th day until you become pure. For further information see Fatwa 83454.
Allaah Knows best.