util.printf

i am new to this game and climbing the wall.

i using AcrobatPro9 in windows.

i want to create a text field that:
has format category: number
decimal places: zero
if another text field (same format) is zero then print zero no decimal places
else a calculation with result to 4 decimal places

i have tried to use "util.printf " it is not working:
var n = 8.1234567;
event.value = util.printf("% .4f",n);

i get an error message:
"the value entered does not match format of field"

if i set the format - decimal places to 5 and try:
var n = 6.123456789;
event.value = util.printf("%.2f",n);

i get:
6.12000

why i am not able to change the decimal place format?