The fourth season of Supernatural is generally agreed to be one of the best, when the show was really hitting its stride. Season four starts with Dean coming back from his time in Hell, and manages to balance out some great monster-of-the-week episodes (and some that are not so great), as well as a handful of truly hilarious ones, as well as introducing some characters and plotlines that will remain for the rest of the series.
Season four brings Castiel into the mix, and moves from the early arcs about the Yellow-Eyed Demon to some of the larger ones that will come into play for seasons to come - not just Castiel, but Adam (the third, and oft-forgotten Winchester brother), Chuck, and the Supernatural book series that the show returns to for some of the best episodes in later seasons, too. It marks a change in the show, when much longer arcs start to evolve, and ends with the arrival of Lucifer himself - but which episodes were hits, and which just didn't measure up?
Best: In The Beginning (9.3)
The title says it all in this episode, where Dean is able to go back in time (not for the last time) and meet his parents when they were young and in love. He learns that his mother (and her family) were hunters too, and that she is intending to get away from the lifestyle with John (not that that turned out well...). There are some sweet and emotional moments between Dean and his young mother in this episode, but the kicker has to be the reveal that his attempts to stop the Yellow-Eyed Demon from getting to Sam just end with John dead, and the demon making a deal with Mary to bring him back - the same deal that ends up with baby Sam being fed demon's blood.
Worst: Monster Movie (8.5)
Three episodes from this season manage to share the same rating of 8.5: 'Monster Movie', 'Sex and Violence', and 'It's The Great Pumpkin, Sam Winchester'. None are truly awful, but they simply fail to wow as much as the rest of the season. 'Monster Movie' sees the boys hunt a shapeshifter who likes to appear as classic movie monsters, 'Sex and Violence' sees them tangle with a siren, and 'It's The Great Pumpkin, Sam Winchester' has them taking on a witch who is killing to summon a demon.
Best: Yellow Fever (9.3)
This may be one of the most beloved episodes of Supernatural ever - and it's led to one of the most popular gifs, of the macho Dean Winchester shrieking in fright! The episode has the Winchesters taking on the strange case of people seemingly dying of nothing more that sheer terror - and they learn about 'ghost sickness', a paranormal disease that kills people with fear.
Dean himself contracts it, and becomes increasingly terrified of everything (even a teeny tiny doggie!). 'Yellow Fever' may not have huge plot or emotional impact, but it's one of the funniest episodes the show has ever made.
Worst: Jump The Shark (8.4)
This episode is full of twists and turns, and introduces a character that seems minor at this point in the series - but fans will know he makes a return more than once before Supernatural ends. That character is, of course, Adam Milligan, the surprise half-brother of Sam and Dean. At first, the boys assume he is lying, only to find out that he's actually telling the truth (and John wasn't quite as pure and good as they thought)... only to then find out that while Adam Milligan did exist, the version of him they were talking to was a ghoul, not their brother, who is already dead.
Best: The Monster At The End Of This Book (9.3)
It's no surprise that this episode is one of the best of the season - and it's actually one that sets up a fan-favorite plotline for years to come, as well as introducing one of the most important characters of the final seasons: Chuck. The episode starts with Sam and Dean investigating a case, as usual, but being mistaken for cosplayers... of themselves! They discover that there is a book series, written by Carver Edlund, that documents their entire adventures thus far. Not only does this bring in Chuck and the 'Supernatural' book series, but it's one of the more meta episodes of the show, which is always a pleasure to watch.
Worst: Metamorphosis (8.2)
This is a complicated and heavy episode, which may be why it is one of the least popular of this season. While Supernatural gets all kinds of dark in the later seasons, at this point in the show the comedy element is definitely stronger, and 'Metamorphosis' feels a little out of place as a result. The episode centers on a man who is becoming a Rugaru (a cannibalistic monster) without realizing, and the struggles of deciding when man becomes monster, and how to deal with that change.
Best: Lucifer Rising (9.4)
This intense season finale is undoubtedly deserving of its spot as one of the best two episodes of the series, and it introduces another huge fan-favorite character who will have an impact on the series for seasons to come: Lucifer himself.
The finale starts with the Winchesters trying to avert the apocalypse (of course), but failing due to the machinations of Ruby, and releasing Lucifer himself on the world in the kind of massive cliffhanger finale that Supernatural is known for.
Worst: Family Remains (8.1)
Another dark episode, 'Family Remains' is a fairly straightforward monster-of-the-week style look at a haunted house, with a new family moving in. Overall, it's just a bit bland by Supernatural standards, with the Winchesters discovering the mystery of the ghost, saving the family, and destroying the monster in the end.
Best: Lazarus Rising (9.5)
The season premiere 'Lazarus Rising' picks up a few months after the previous season finale, as Dean is brought back from Hell by a mysterious force. The episode is packed with emotional moments as Dean reunites with Bobby and Sam, and they search out whatever brought him back - but it's also the episode that introduces Castiel to the show! For long-time fans, it may be hard to remember a time before Castiel was a major character, but this is where his story really begins.
Worst: Criss Angel Is A Douchebag (8.1)
The lowest-rated of the entire season, 'Criss Angel Is A Douchebag' is another fairly formulaic monster-of-the-week episode, as the Winchesters deal with a trio of old-school stage magicians who are struggling to deal with being replaced by newer, flashier entertainers. Real magic comes into play with a deck of enchanted Tarot cards, and the boys have to save the day, as usual. While not terrible, it's simply not that great.