The Evo has been getting tons of reviews lately, and pretty much all of them say the 4G kills the battery fast. A few reviews even say Sprint is planning on educating customers to turn 4G off and only turn it on when needed.
Check out the Engadget and Wall Street Journal reviews. They both talk about how 4G affects the battery life:
http://www.engadget.com/2010/05/19/htc-evo-4g-review/
http://online.wsj.com/article/SB1000...850349228.html