Tiered Credit Terms (Payment Discounts)

We have a couple of customers that require tiered discount terms.  One of them is a very large international company, so I’m sure others have ran into this situation as well.  If I have a customer that wants us to allow 4% discounts at Net 10 days that then trickle down daily to a 0% discount when the invoice hits 120 days, how do I make that happen?  The only way I can see it working is creating terms that would allow the customer a 4% discount for paying at 120 days since the system will not allow for a discount to be even a penny more than listed when the invoice is created, in addition to there only being one discount date available on my screen, not multiple due dates.  I just don’t know how to make this so that the customer doesn’t have the ability to take a 4% discount no matter what.

In addition, I have a customer that wants their tier to be 4% N30, 3% N60, 2% N90, 1% N120, 0% after that.  Again, how do I accomplish this without the customer having the ability to take a 4% discount every time?