It seems to me that you already know what testing will say, because your DD is already showing abilities that correspond nicely with your own IQ. For similar reasons, I found that testing for my DD merely confirmed what we already knew.
Any testing would only be of a benefit in proving your DD's abilities to someone else, like a school principal, in order to gain her access to services that better fit her needs. In this case, the school is likely to trust their own process/resources, so outside testing is often a waste of time and money.
My advice would be to start a dialog about your DD's abilities with the school. The determination to test or not would be an outgrowth of that.
As for the notion that all kids level out by 4th grade: that's a red flag. Anyone who says it clearly doesn't understand differences in ability. If you made the strongest kid in school sit on a stool all day, while the rest of the class pumped iron for hours, in a few years he won't look so strong compared to to his peers... unless the kid secretly works out at home on his own. That's pretty much what they do to our children in K-4.
