DD9's Lexile in 2nd grade was higher than in 4th grade. Go figure. It's a rough guide at best.
What we do is let her have access to anything she wants to read. Her teacher did this as well in 3rd grade. Instead of her reading stuff at her Lexile level, it usually ends up being grade-level stuff like Harry Potter, Hunger Games, and the "Goddess Girl" series. She is not voracious in that she is very particular about what she reads, especially if it contains content that upsets her.
Then we have her try bigger books that are in the upper end of her range. This is hit or miss but in the past few months, thanks to public domain books on her Kindle, she has read and enjoyed unabridged versions of:
Little Women
Little Men
The Secret Garden
Grimm's Fairy Tales
Treasure Island
I am also encouraging her to use the Kindle's built-in dictionary function whenever she hits a word she doesn't know, but this is proving a challenge to get her to do it consistently.
