As y'all know he drowned at the end of Season 3, after speaking with Penny Widmore. It's that dratted Mikhail: he detonated his stupid grenade and flooded the Looking Glass station's communication room.
Anyway here's what Entertainment Weekly says about it:
''So much for fate.'' Those are Charlie's last words — at least in this dimension. Confident that he'd beaten Desmond's prophecy of his death by drowning, he turns to see the eye-patch-wearing Other he thought he'd killed blow up the substation's window. As the seawater rises to submerge him inside the Looking Glass station, Charlie slams the door to protect Desmond and scribbles a note of warning to him that the people the castaways think are coming to save them are not their friends. Charlie might not have beaten fate, but he beat his demons, as the heroin-addicted rock star finally had his redemption.
Gosh, I miss Charlie! Do you think he'll appear in Season 5 or 6?
Anyway, check out the photo gallery if you like.