I have had my D4 for over a month now and traveled across the country with it and my sensor only showed two small dust specks. I used the mirror lock up for cleaning option and with a few puffs from my Giotto Rocket blower the specks are gone. The built in mirror cleaning did not work for these specks. I have only had to clean my D3s with the built in cleaning cycle or with a blower in all of the years I have owned it. I shoot a lot of horse racing on dirt tracks and even with the dust never had any issues with a dirty sensor. I would try the built in sensor cleaner before doing anything.