The interest rate is 16.59% per year compounded yearly. To be consistent, let's convert the unit of time to year.
Time = 18 months (1 yr/12 months) = 1.5 yrs
Then, we use this formula:
F = P(1+i)^n
F = 4308(1+0.1659)^1.5
F = $5,423.35
The balance would be $5,423.35.