Your knee has to always be covered. Period. I do’t agree with the whole 4 inches thing because people make it out to be a halacha. Which it’s not. (As far as I know)
It’s really different for every person. For example, someone mentioned above that as long as the hem hits halfway between your knee and your ankle, you’re fine. That would never work for me. So just cover your knees at all times and you’ll be fine. I really don’t know why people really measure 4 inches.