We had a tranisition plan written by the school for DS' subject acceleration and then his whole grade skip. It outlined details like where he'd sit, who would take charge of making sure he knew the routine (could be a 4th grader in your case), and which administrators would monitor his progress and at what frequency.

For 3-> 4, it might be useful to meet the teacher first so that the teacher can think about buddying her up with another girl in the class for a short while.