in the eCart object, create a new column named shippable.
In the add to cart button behavior, for shippable items set this column to 1 for digital items, set it to 0
in your shipping rules use the trigger,
Based on Items with a specific value:
If the total number of items where 'shippable has a value of '1' is '>' a value of '0'
If the total number of items where 'shippable has a value of '1' is '<=' a value of '4'
Flat rate charge of 11.50
then create another rule for 4 - 8 items using the same trigger, just changing the comparison values.