Coming into the fourth season, fans of "Supernatural" had reason to be concerned. The ratings for the third season had been challenging, to say the least, and the competition was fierce. With the CW Network encountering more and more difficulties in its bid to stay alive, there was reason to wonder if the executives would decide that canceling "Supernatural" and replacing it with something "fresh" would be an option.
Those same fans, like the critics and the television pundits alike, were stunned when the numbers starting pouring in for the fourth season. "Supernatural" has accomplished something truly unusual coming out of the writers' strike: it gained back a number of viewers that it had appeared to lose. That kind of latter-season growth is almost unheard of, especially for a show that has little or no promotion compared to its competition.
It's not as simple as pointing to a rabid fanbase that talked up the show during the summer hiatus in 2008. That might have explained the initial bump for the season premiere, but it wouldn't pertain to the episodes that followed. Those numbers would be much more related to the viewers' reaction to the material they had already seen. And that means that the ratings growth was a sign that the audience liked what they saw. A lot.
And that comes as no surprise to the fans who have stuck with the adventures of the Brothers Winchester from the beginning. Eric Kripke has been operating with a plan, and even the writers' strike has done little to derail it. From the very first season, there were stirrings of a great war against demons to come, and regardless of how low the budget might get, Kripke has stayed that course with little deviation. (Note how the budget remains pitiful, despite the rising favor granted to the show by TPTB.)
This season expanded the mythos of the "Supernatural" universe by introducing angels to what has been, in the past, a fairly demon-ruled spiritual realm. The introduction of Castiel in particular has been incredibly popular. The season arc was a combination of high-level plot threads grounded in character development and advancement, which is essentially the same formula that has gained the show so much storytelling success.
The plot is relatively simple. Lilith, Sam's nemesis within the demonic horde, is trying to open 66 seals necessary for the release of Lucifer. The Brothers Winchester need to stop that from happening (and the reason why is one of the best reveals of the season). Unfortunately, Sam and Dean are not on the same page as to how to stop Lilith.
Sam, right from the beginning, is under Ruby's influence, and considering that her gambit had been initiated back in the beginning of the third season, it's easier to understand what is happening in his case. The writers had been setting up Sam's descent into darkness since the second season, so seeing it happen was all the more disturbing.
Dean, on the other hand, is fresh from the bowels of Hell, at the hands of Castiel, and his purpose is a lot more mysterious. It's clear from very early on in the season that he will end up opposing Sam by the time the mission comes to a head, but his relationship to the angels is sufficiently complicated to keep things in doubt. It is this dual manipulation, driving the Brothers Winchester apart, that makes the season so compelling.