Am I to consider the four days as menstruation and perform ghusl after all colors disappear, knowing that my menses used to be four days, then became a day and a half followed by dryness, then yellowish or brownish discharge, then complete dryness, then a light yellowish discharge on the fourth day?

The preponderant view is that yellowish or brownish discharge is considered menses if it occurs during the usual period. If a woman sees purity, she must promptly perform ghusl. So, if she sees purity by dryness, she performs ghusl. If blood, or yellowish or brownish discharge returns within the period of her usual cycle, she is considered menstruating again. She then performs ghusl once more when it ceases. Some scholars have allowed waiting for a day or a day and a half to confirm purity.