$100,000 for 20 years compounded at 4 percent annually results in a rate per period of:

4% x 20 = 80% (in 20 years)
but the 80% of $100,000 is $100,000 x 80 /100 = $ 80000
so 80% for $ 80000 (20 years)
     4%   for   x=?
the answer is  x= 4000$
$100,000 for 20 years compounded at 4 percent annually results in a rate per period of: 4000$