I've been almost everywhere on the Outerbanks, a few things you need to think about.
As mentioned, September is hurricane season. It's unlikely that one will happen but not out of the question.
If you're going after Labor Day rates will drop and you should have good options on accommodations. There aren't any huge hotels in the outerbanks and almost everyone rents houses. Look on VRBO, HomeAway, etc. or local realtor sites.
Weather should be nice, however in September things start slowing and shutting down. The later into September and October you get the more you should expect shops, restaurants, etc. to be closed. Also expect the beaches to be pretty much empty.
Four days is not enough in my opinion, especially depending on how you get there and where you are going. Many parts are big drives, especially Hatteras and Ocracoke. I generally won't go if it's less than a week.
If she wants the Nicholas Sparks experience she's talking about Rodanthe, so I'd look there first. Nags Head is the most busy and accessible (although least appealing to me) but may be a good option that time of year, Ocracoke is the least accessible (Ferry only) but super cool. For the time of year you're going I probably wouldn't stay north of Duck, there isn't a whole lot to do up there and some of it is 4WD accessible only.