Ok, thanks, this gives me enough to go on.
add new columns to the cart object for each of the discount amount:
discountA
discountB
discountC
discountD
in the add to cart button, give these columns the discount amount for that items type, so for type 1:
discountA = 2.50
discountB = 3.90
discountC = 5.00
discountD = 6.00
for type 2:
discountA = 3.30
discountB = 5.00,
discountC = 6.00
discountD = 8.00
etc....
then create the TotalItemDiscount calculation as:
((abs([Quantity] >= 100 && [Quantity] < 500)?[discountA]:0) + (abs([Quantity] >= 500 && [Quantity] < 1000)?[discountB]:0) + (abs([Quantity] >= 1000 && [Quantity] < 2000)?[discountC]:0) + (abs([Quantity] >= 2000 && [Quantity] < 4000)?[discountD]:0)) * [Quantity]
then define the discount on the discount tab as:
Trigger: Number of unique items in the cart
Calculation: Based on multiple of column subtotal
Subtotal of column "TotalItemDiscount" "times" "1" "plus" "0"