Dried up Nipple?

Emma's kittens are 4 days old and I noticed just now that one of her nipples is dried (almost looks like their umbilical cords) and the mammary is full and hard. It doesn't seem to bother her - no pain or oozing or anything. Is that normal? She only has three kittens, so obviously they're all ignoring this nipple. Do I need to do anything? Of course, I'll call the vet in the a.m. but wondered if there was anything I needed/could do in the meanwhile.
Thanks!

A warm, damp compress is sometimes appreciated but your mileage may vary - some queens enjoy it, some don't. I concur that a vet visit is indicated ... you want to rule out mastitis.