According to this forum thread, if you set AllowRounding="false", then it will disable the ability to display trailing zeros after your decimal point.
In this forum thread, near the bottom, the following solution is marked as the answer to keep 4 decimal digits, including the trailing zeros, without allowing rounding:
In order to show 4 decimal digits, you have to set AllowRounding="false" and KeepNotRoundedValue="true"