These spoilers are from SpoilerFix.com. Also, I took off the ones for tonight's episode:

Episode 4.20: The Rapture
Airdate: April 30, 2009

~ Castiel appears to Dean in a dream and says he has something important to tell him but they need to meet somewhere private. Dean and Sam go searching for Castiel but instead find Jimmy, Castiel's human vessel, who has only a spotty recollection of being an angel. Jimmy wants to return to his family and normal life, but Dean and Sam are concerned about his safety. Source: The CW

~ Lilith will likely raise her adorable little demonic head again in episode 20. Supernatural is casting for a suddenly possessed cutie-pie 11-year-old who turns into an evil ice queen. (And that's Lilith if I've ever heard her described.) Source: Kristin on E!Online

Episode 4.21: When the Levee Breaks
Airdate: May 7, 2009

~ Dean and Bobby lock Sam in Bobby's panic room so he can detox from the demon blood. However, as Bobby sees more seals are being broken he tells Dean they should let Sam out to help them fight the impending apocalypse. Dean disagrees and goes to Castiel for help. Sam and Dean have a big blowout confrontation. Source: The CW

~ The boys will hear from Rufus in Episode 21, which is titled "When the Levee Breaks" and airs May 7. Source: TV Guide Online

Episode 4.22: Lucifer Rising (Season finale)
Airdate: May 14, 2009

~ We just chatted up SPN writer Ben Edlund, and he tells us that Lucifer will be more of a looming presence than one specific personage. Source: Kristin on E!Online

~ Ausiello updated his asterix hint to this: Sam learns in the season finale that ******i** ****** is the ***** **** that has ** ** broken ****** ******* can ***e. Source: The Ausiello Files [Note: Site visitor Brandon believes that a possible answer to the riddle is "Sam learns in the season finale that murdering ****** is the final seal that has to be broken before Lucifer can rise." A few site visitors believe that the person who must be killed is "Lilith". Site visitor Courtney emailed me with a version of the possible answer with "defeating" instead of "murdering" as Brandon suggested.]

General Spoilers:
~ Comic book geek-slash-prophet Chuck will be back! Source: The Ausiello Files

~ While Castiel (Misha Collins) is confirmed to be a regular for Supernatural season five, all we can say about Anna and Ruby is that there are a couple deaths in the finale, and that Sam's demon girlfriend and Dean's angelic friend with benefits are not guaranteed to be safe. [...] We do see John Winchester on Supernatural in photographic form before the end of the season. Source: Kristin on E!Online

~ Maybe [the big Winchester brawl that's going to happen in the season finale] has something to do with Sam's demon blood addiction. Or maybe it's fallout from a certain death that's coming up in April. Source: The Ausiello Files

~ Let's just say Sam may need to start going to meetings. The kind of meeting where you step up in front of a room and declare, "Hi. My name is Sam, and I'm a Demon Bloodholic." Source: The Ausiello Files

~ The final three episodes of the season can be summed up in three words: Dean vs. Sam. Source: The Ausiello Files