I was wondering if wearing a skirt 4 inches below the knee is halacha or just a chumra
The idea of wearing the skirt 4 inches below the knee, is brought in numerous contemporary poskim. The idea being that the halacha says that woman’s thigh is considered ervah and must be covered at all times. The thigh ends at the bottom of knee cap, (not the top). If a woman will wear a skirt that technically covers past the bottom of her knee, technically it would be alright, however when she picks up her foot such as when going up the stairs or getting into a car, if when crossing her legs while sitting, it will cause her skirt to ride up, and her thigh will be showing in the front or in the back. Therefore if her skirt will be 4 inches below the knee, this will ensure that she is tzniusdik in all positions.