This is known as a monthly annuity.
You will need a special formula (See attached).
rate = 4/1,200 =
<span>
<span>
<span>
0.0033333333
</span>
</span>
</span>
number of months = 12*12 = 144
Monthly Amount = Total / ([(1+r) ^ (n+1) -1] / [r]) -1
Monthly Amount = 81,000 / [(1<span>.0033333)^(145) -1] / </span> <span>.0033333333] -1
</span>Monthly Amount = 81,000 /
<span>
<span>
<span>
[1.6201675397
</span>
</span>
</span>
-1] / [<span>.0033333333] -1
</span>Monthly Amount = 81,000 /
[<span>.6201675397 /</span><span> .0033333333] -1
</span>Monthly Amount = 81,000 /
<span>
<span>
<span>
[186.0502637705 -1]
</span></span></span>Monthly Amount = 81,000 /
185.0502637705
Monthly Amount = 437.71
Here's a monthly annuity calculator to check your work:
http://www.1728.org/annuitym.htm