I do prospect research in the development office here, and we're always trying to find new potential donors. Having more granular information would be more helpful than the general sources I've been using (the Community Sourcebook of ZIP Code Demographics from 2007, various websites).
Currently, we do not provide demographic data to our address verification service. We have looked at adding that to our service but it is not in the works yet. Being able to determine population data and income (gathered by the census) on a ZIP+4 level would be very valuable to anyone trying to analyze statistics and trends.
We provide the validated address and you can then apply the ZIP+4 data provided by other services. That would enable you to have accurate statistics. One company that provides ZIP+4 demographic data is: zip-codes.com (it's in their ZIP+4 section)