Certain LS4s are prone to having their EVAP Canister Purge Solenoids gum up and stop working.  This solenoid opens and closes the valve to the charcoal canister that contains fuel vapors that would otherwise be released into the atmosphere.  When the valve no longer works properly, the Check Engine Light is illuminated and the P0445 or P0449 is stored in the logs.  

Actually replacing this part is a very simple job--disconnect the wiring harness, unclamp the two hoses attached to the solenoid, remove one 10mm bolt, and the solenoid comes right out.  It's up under the car behind the driver's side rear wheel.  In fact, you can reach it without having to even jack the car up.