Most four year olds cry when they receive their vaccinations, so it was probably no surprise when Dr. Nguyen's staff heard her consoling behind a closed door, "It's okay sweetie, don't cry,"  but this time she was talking to me.   "Which do you want to do this time, the polio or the MMR?" she had asked because we've been breaking them up, trying to be precautious with a slow vaccine schedule sinc…