For payments of $402 you will pay it off in 26 months according to
this site .
Over such a short time frame, interest (especially a low rate like 4%) isn't really going to have much time to compound on you so aren't really going to add many payments.
Here is the formula for calculating a payment:
P = principal
r = interest rate
m = length of loan in months
P ( r / 12 )
-------------------------
(1 - ( 1 + r / 12 )^-m )
Feel free to try and solve for m, but it looks like some nasty math with logarithms.