(e.g. four inches)
That isn’t a geder. You have to cover the knee. However, the knee must be covered even when sitting, walking, and getting into/out of a car as well, and if your skirt just covers your knee while standing straight it will not cover it in other positions. Therefore, some length has to be added. There is however, no Halachic quantification for this. If you need 4 inches, then it’s 4 inches. If you need 3, 6, or 8, then that’s what it is.