Fact 1: The knee is an ervah.
Fact 2: An ervah must always be covered at all times.
Fact 3: The lengths quoted like 4cm are advisory lengths for insuring Fact 2 (that the knee remains covered in all positions at all times).
Fact 4: Unfortunately, too many knees ARE exposed in the sitting position, sometimes with the wearer constantly tugging in vain at the bottom of the skirt as if it were made of a stretchy material in a hopeless struggle to keep the knee covered when men are near.
Fact 5: Just as women cannot understand a man’s yetzer hora, a man does not understand the yetzer hora of a woman so we cannot criticize but must nevertheless make sure that halacha is always upheld, especially in our own homes where the man must bear ultimate responsibility.