Is the brown blood that appears a week after the end of menstruation and lasts for four days, accompanied by a burning sensation, considered menses or istihada (non-menstrual bleeding), especially given the presence of an allergy that prevents distinguishing the odor, and is it necessary to make up the fasts of these days if they occurred during Ramadan?

If the discharge is brownish (kudrah), your fast is valid, because kudrah is not considered menstruation unless it occurs during the usual period of menstruation or is connected to blood. However, if it is explicit blood, then the rulings of menstruation apply to it, provided that its duration and the total of blood and purity periods do not exceed fifteen days, and making up the fast is obligatory. If it occurs at a time when it cannot be menstruation, then it is considered istihadah (definition) (non-menstrual bleeding), and the fast is valid, and ghusl (definition) is not required.