Avg() or round() : How to limit the decimal place?

Hello,

The avg() function im statement returns values like that:
<avgDelay>48413.448275862068965517</avgDelay>

If I use round() so I would get 48413.  But how do I get 48413.4483 ? How
can I limit the decimal place to 4?

I couldn't find anything in W3 Draft.

Thanks
Houman