Gene Milton borrowed a sum of $5,000 from his rich uncle Ben. After three years, Gene paid a sum of $4,000 towards the accumulated interest and partial principal. He also paid a sum of $2,500 at the end of five years to pay off the loan. The interest rates quoted by the rich uncle were two-tiered. The interest rate for the first three years was 6% compounded yearly and a slightly higher rate for years four and five. What was the interest rate for years 4 and 5?