Car not starting after cel earlier

Okay everyone... So heres the story. Yesterday I was driving home and the CEL popped on. It was a Bank 1 system too lean. Not a big deal.. I have an Injen SRI so I figured it was that or a funky reading from a sensor. Anyhow I left it parked for a few hours and went to start it up again and it wont start. The door ajar beeping is all slow and funky.. The lights are dim and when I try to start it, the dash lights flicker and it makes a bunch of click noises.

Battery is reading 12-11 before I try to start it up.. Then drops to like 4 after.
The battery itself is clean.. No build up on terminals.
The ground cable from intake manifold to firewall looks in perfect shape.

Any suggestions or comments?

Thanks!