If health insurance copay for generic drug is more than the price of drug what do you pay?

I am looking at medicare supplement plans. Most plans say they cover generic drugs. The copay for one plan for a 30 day supply of a generic drug is $10. Suppose the generic drug is one that Wal-mart charges $4 for a 30 day supply. What do you pay; $4 or $10?
--------------------
The way my plan works is you pay the copay or the cost of the drug, whichever is less. For example, I have a $10 copay for generic drugs. I recently filled a prescription that only cost $8, so I only paid $8. I'm pretty sure this is the way it works with most plans, but it's always a good idea to double check. Contact the customer service line for the plans you are interested in. Good luck.
Source