This is pretty typical and does not necessarily mean your child "has a stutter" that is going to stick. It means, especially if she's "beyond her years speech-wise" as you say, that she is on the verge of a leap forward in communicating, and knows what she wants to say but not quite how to say it, and is so focused on and excited about talking that she can't quite spit out the words. I think a lot of, if not most, kids do some of this at times.
As always, ask your pediatrician and/or her preschool teacher if she has one, if you're worried. I think they'll tell you to wait it out and be very patient with her speech, and not try to guess at her words but let her get out what she wants to get out in her own time.