“As far as the inches, there is no hard halachic number. Some people will need even more than 4″.”
I have a coworker whose skirts practically drag on the floor when she walks, yet when she sits at her desk, that’s not long enough.
In other words, ladies use your judgement what works for you and the way you live your life (IE stationary, constantly on the move, whatever) is how you should dress.