Nicky,

Welcome! The links shared by mama2three are great, and I might add one more which deals not specifically with testing but with the dilemma you mentioned as to whether children tend to level out by 3rd or 4th grade.

http://giftedkids.about.com/od/schoolissues/i/even_out_2.htm

NOTE:
the above link now redirects to
https://www.verywellfamily.com/is-my-child-gifted-1449131