First, plan plenty of time so there won't be added stress of having to rush. That way you don't have to be worried about getting through security, bathroom breaks and to the gate in a short time frame. Get there early and make sure she has a bathroom break before boarding. (But, if she is like my 4 year old, she is gonna want to go to the bathroom on the plane no matter what!)
Oh - you may want to have her wear shoes that are easy off and easy on, tennis shoes may be a bit of a hassle through security.
I packed a small backpack for her. It included anything I could think of to help her pass the time. We had coloring books, crayons (in a ziplock bag), portable DVD player (with headphones), DVDs, books. I also packed a big kid sippy cup (empty but I purchased juice and a bottle of water once I went through security) and ziplocks of snacks - grapes, carrots, crackers, raisins, any of her favorites but limit the sugar cause you don't want a hyper girl on that plane. If she has a lovey, don't forget that.
I also would dress her in light layers, you never know if it is going to be hot or cold on the plane. And, just in case, a spare pair of panties and pants - she may have an accident due to the excitement. I also took some wipes, just in case there were any spills, etc. My daughter loves to chew gum, so we bought her favorite kind to help with her ears.
We talked about our plan everyday for about a week before the flight so she knew what to expect and that she'd have to be on her best behavior. We also discussed how extremely important it was for us to hold hands everywhere.
I didn't take a stroller, she wouldn't sit in it anyway. I checked her carseat with our suitcases. The least amount of stuff you have on the plane the easier it will be to keep up with her.
She will probably be more excited than anything. My daughter loves to sit by the window and watch the clouds. We also had a disposable camera that was just for her during our trip, she loved that she could take pictures of anything she wanted.
Most of all, relax. Our kids feel our stress and your stress may make her stressed. Enjoy your trip!