Drilling additional small (1/2") holes above the level of the 4 inch holes accomplishes nothing.
Drilling additional small (1/2") holes below the level of the 4 inch holes will allow desaturation of the soil below the level of the drain pipes but only for a few feet radius around the pit. This is optional. A slight hazard exists that mud may enter the pit through these additional holes.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?