There is a mistake in your printf line, too. Try the following code:
void main()
{
var my_var = 1404.5;
printf("%.4f",my_var);
printf("%.4f",(double)my_var);
}
The first printf will tell you that the variable is "0.0000", the second printf tells you the correct value. Check the manual for printf, it does not accept vars.
