Hey there!
Okay, so writers have confirmed that Charlie is dead on the show. But that doesn't mean he could return in flashbacks, or in the form of the black smoke monster - or in similar ways that Walt, Boone, Ana Lucia, Christian Sheppard have appeared when they couldn't have done as they were either dead excetpt for Walt, but he was contained by the Others - so thats another story.
Anyway, upon watching season 4 trailer in slow mo, pausing it at 3.09 seconds, Charlie can be spotted looking at Hurley, wearing a black leather jacket, taking his sun shades off. To see it, you will need to pause the vid, as it goes to quick to try and spot in detail.
OMG, am so excited! I really hope I am right on this, it certianly looks like Charlie, I really hope it is! Yes, it is him. He's just had a haircut since he did, LOL.
Anyway, now I REALLY can't wait for LOST to return! *SQUEALS*