First off, hows the battery? 4 years is a long time in car life. See if you can jump it. If not, replace the battery. If that doesn't work, post back.
If the battery has a charge, it may be weak enough that it's not starting. Do any of the electronics come on when you turn the key to "ON" just before you start? Turn off all electronics you can think of, like heater, radio, heated seat, etc., to save the drain put on the battery. You can leave the ASC and Traction control's on, they need to be on to run the car's pre-ignition tests.