Here is a loan calculator
FinAid | Calculators | Loan Calculator
If you plug your debt and interest rate in and the number of years over which you will be repaying it it will tell you what your monthly payments will be. And the variable interest rate - any loan where there was even a possibility of an interest rate of 16% would make me run away very quickly.
As Sue said - if you are deferring the interest make sure you add it and compound it to see what your actual debt would be at the end of 4 years.