They are very similar in size. But, the ladies 6 is just a bit longer and wider then the kids size 4
I am a perfect European size 38 (7 to 7 1/2 U.S.) and the Youth size 5 are perfect for me in both Chaco and Teva brands. You just need to be careful that when shopping STP that you are looking at Youth sizes and not Kids. For the majority of people, I believe that a Youth 6 would work for those who wear a Women's size 8 and Youth 4 would work for those who wear a Women's size 6 to 6 1/2.