Dear Everyone

I'm new-bee to ZenCart and have a question regarding Group Pricing+Quantity Discounts.

I have 2 customer pattern, Retail and Certified.
Certified customer can purchase 20% off for any item.(I apply Group pricing for them)
Retail customer has no discounts.

But for only 1 product group, I would like to apply volume discounts function only for Retail customer.
for example, if you buy 1-4, $10 each, 5-10, $8 each.
Only Retail customer can receive this discount, not for certified customer,
because certified customer allready have 20% discounts for all items include this items.

Is there any ways to do this?
I'm using Ver1.3.5.

Thank you in advance.

Michele