There is an 8/16” difference between a B and D width and a 6/16” (3/8”) difference between 4E and 6E. It’s important to note that this is the maximum difference in width. This occurs across the ball of the foot. There is less of a difference as you move towards the tip of the shoe and towards the heel.
There is a 1/6” difference between each ½ size (ie, between 9 and 9.5). For every ½ size up, the width (across the ball) will increase by 1/8 of an inch.