Alright so it really isn't spoilery but it could be and so I'm going to put it under a tag. If you are interested then click, however I know theories tend to make people question, think, and when they come true yell about spoilers. So if you want no outside influence, don't click on the cut. I haven't read any spoilers so this is all in my head (well except the hints I got at Comic Con which wasn't much but enough for this theory).
Here is my ultimate theory of Season 4 and what the hell is going on. It actually ties in with pre-story *GASP SHOCK AND DISMAY* I would get into the Mary Winchester stuff but that's small apples compared to this.
Now I know everyone is afraid of Castiel but really, is he who you should be afraid of? *dun dun dun* You may be asking yourself, what happened to Lilith? Why did Lilith want to kill Sam? Did she just not like him? Let me go back to the pre-story that is in my head.
Once upon a time in hell there was a BIG BAD FALLEN ANGEL NAMED LUCIFER (who we have already heard about in Sin City from the demon with a heart, Casey). Now the other demons and such in Hell went... you know... we don't like you leading us anymore... and we are demons... power hungry demons. So they trapped him somewhere in a very very hard to release place or by some magic means.
With this power vaccuum in Hell there was a demon named Azaziel who went... I'LL LEAD YOU ALL! And so he became the big baddy in town. Not all demons rebeled against Lucifer. There were some who by "faith" believed he still existed and went about trying to find him. YED went... SCREW THAT and decided to make an army of special children. But... he only needed one in the end because really, when you have your personal body guard that can kill Lucifer if he ever does return, you don't need 20 of them. Plus they'll get all power hungry so you just keep the best of the best.
Unfortunately YED died but his personal guard was left alive (HI SAM!). Lilith saw this as an opportunity to bring back ye olde Lucifer for real. Of course... getting rid of Sam would be great! No threat there! (of course she's failed but her whole goal last season wasn't for power but to get rid of the one obstacle in her plan to get Luci back).
Castiel I'm going back and forth on between two things. He either is a) a misguided angel who THINKS he is doing the work of God or b) he is in cahoots with Lilith and Lucifer. If A he is a victim, if B he's a bastard. I believe more in the B route of him being a fallen angel in cahoots and that his brought Dean back to pit him against Sam. Will convince Dean that Sam is EVIL and therefore must be destroyed when in reality Sam is the only weapon of good left and is doing the right thing. Whether Dean does the will of Castiel is a whole other issue.
So in conclusion... Lucifer is coming, end of the world, Sam can stop it but Dean is being used to stop Sam.