I assume you mean moonlight as a source, not the moon as a subject?
Either way, it's going to depend a lot on what you want your image to look like. A lot of movie and photo tend to go cool to give that "night feel". It's also going to be warmer when the moon is close to the horizon. That said, I'd start at 4200 or so and adjust in post to fit my mood.
Edit: well, Wikipedia says 4100, and Wiki never lies.