in the eCart object, create a new calculation, name it "shipRate" as:
((([Price] * [Quantity]) * 0.10) > 4)?(([Price] * [Quantity]) * 0.10):4
this basically says
If the TotalPrice times %10 is greater than 4, use TotalPrice times %10. If not, use 4
Then on the shipping tab, create a new shipping rule. Set the trigger to Total Number of Unique Items in the cart ">" "0"
and set the calculation to use "Based on column subtotal"
Subtotal of column "shipRate" "plus" "0"
see the included screen shot