Hi,
I may be wrong, but I think your formula to calculate the monthly interest rate, is wrong.
Using your example: 4.3% per year and initial amount of $1500 means after 1 year: A1 = 1500*1.043 = 1564,50. Your formula gives: A1 = 1500*(1+0.043/4)^(4*1) = 1565.55 (rounded).
Being a mentor I let you find out yourself, how to fix this ;)
Can you fix it? otherwise I will :P