Simple interest:
i = prt
where i = interest earned
p = principal deposited
r = annual rate
t = number of years
You want the interest to be $500. The rate is 4%, and the principal is $2000.
Enter those values in the formula for simple interest above and solve for t, the time in years.
500 = 2000 * 4% * t
500 = 2000 * 0.04 * t
500 = 80t
t = 6.25
Answer: 6.25 years