He probably decides based on what each person is ready to accept. Force 4 inches onto some women and it will push them off the derech. For those, it’s better to leave 4 inches as an ideal to aspire to when they are ready, and only then. I assume that this rav only makes these suggestions to women he knows well (otherwise he would have no idea of when they are ready).
I know a rav who works with baalei teshuva on a regular basis. He kashered one man’s kitchen and the guy was ready to totally accept kashrus, but the rav actually said it was ASSUR for him to take it on all at once. He said to him, right now you’re crawling. I am kashering your kitchen and I expect you to keep it kosher, but I also expect you to eat anything you want outside of your house. He knew that despite this man’s initial enthusiasm, if he were to try to go from completely treif to completely kosher all at once it would be too overwhelming. Therefore he insisted on baby steps. I heard this story many years ago and that quote “right now you’re crawling” has stayed with me and helped me through a lot of tough times.