Happygirlygirl – the halacha is that your knees have to be covered at all times. From what I understand, the Gedolim pointed out that this is impossible unless your skirt is at least 4 inches below your knees. Depending on the skirt, it might have to be a lot more actually, but it is impossible to find ANY skirt that is less than 4 inches that will cover your knees.
Basically, they are just letting us know that according to the Shulchan Aruch, your skirt must be at least 4 inches below your knees.
Personally, I have never taken a ruler and measured a skirt in my life, and I think that people who are into the 4 inches end up inadvertently being over on one of the only clear-cut halachos of tznius (that your knees have to be covered at all times), since most skirts must be more than 4 inches in order for your knees to be covered at all times.
When I try on a skirt, I stand in front of the mirror. I then pull over a chair and sit in front of the mirror. I then cross my legs in front of the mirror. When possible, I will also go to the car and have a friend check if my knees show when I’m getting into the car.