Let's clarify your question. A NW 4 means that the frontal zone, not just the hairline is gone and a small patch at the crown as well. Are you asking how many grafts are needed to redbuild the front? The general rule of thumb is that you'll need about 1000 grafts per NW level to achieve a good result but the reality is that this can swing plus or minus from patient to patient. With FUE you can get away with fewer grafts because follicular units with higher numbers of hairs can be targeted to get better coverage so the the procedure can also factor in to the equation.