Dean and Castiel, Sam and Ruby. The relationships of each brother with a supernatural being, was a major focus of Season Four.
There are many parallels between these relationships. They are each uneasy alliances at best. Ruby and Cas only have any foothold at all because they save a Winchester's life, and because they present as a useful, even essential ally, in the fight to stop the apocalypse.
That alone, I think would only have got them so far. But in Season four, the boys are separated first by death, and later by their own struggles which put a distance between them, leaving a space that the unworldly creatures can wriggle into.
Both Ruby and Cas present as being on their respective Winchester's side, although both are deceitful and manipulate the boys to their own ends. But while they both have their own aims, there is no doubt that, despite themselves, they develop an attachment of sorts to the Winchester boys.
The converse is also true. Sam and Dean both think they are using Cas and Ruby for what they want. And despite themselves, they do develop some trust for in them.
But I think more than this, Ruby and Castiel each symbolise a part of themselves that Sam and Dean are stuggling with.
In the case of Dean, I think Castiel represents Dean's own struggle to move from the obedient, good son and soldier, to a more autonomous person. This isn't a recent development, we really saw it emerge near the end of Season One. Dean of course grew up seeing only two roles in life - you be John Winchester or you could follow John Winchester. Dean did both in a way, was the loyal son, and aped his father's beliefs, and more superficial characteristics (his music, his appearance).
Castiel in Season four starts out as the good soldier, the unquestioning son. But increasingly he sees the rules he follows as rigid and uncompassionate. As Dean has, he starts to question the framework by which he has judged what is good and right. He struggles to make his own decisions, and in that wonderful scene, pleads with Anna to tell him what to do. Dean of course, has asked the same of Sam more than once (including when Sam was dead!). Of course after this episode Castiel does act according to what he thinks is right, only to be punished by Heaven.
Following this he returns to being heaven's blunt instrument - freeing Sam, demanding Dean's fealty, and betraying Anna. It is only in the final episode, that his connection with Dean gives him the motivation, and the strength, to act according to his own precepts.
Similarly, it has been Dean's connections with people - primarily Sam but with others like Bobby as well - that have helped him become his own man. Importantly, Dean doesn't become his father - he recognizes John's traits (esp when he sees them in Sam in "Jump the Shark") as no longer things he wants to emulate.
What part of Sam does Ruby represent? I think Ruby is that part of Sam which wants to be recognized as important in his own right. Sam felt growing up that what he wanted, what he desired, wasn't valued in the Winchester family. By logical extension, that he wasn't. We know from "Bugs" that he thought Dean was John's golden boy. He left the family and even when he gets great grades at school, is going to be a lawyer, doesn't think that will be valued.
And of course we've seen again and again, Sam want Dean to see him as more than just the little brother to be looked after. He wants Dean to trust him, to value his input. I think the irony is that Dean does – look at all their hunts – but because Dean also continues to treat Sam as a 'precious object' to be looked after, this is undermined.
So Ruby is the part of Sam that wants to show everyone what he's capable of, show people who they are. More importantly I think is to find out who they are for themselves. Both I think have a rather fragile sense of self, and when they find an Important Mission, this gives them the meaning and purpose they are searching for.
At the end of S4 where are these avatars? The situation is more hopeful for Dean as Castiel has resumed his journey to selfhood. I think though he needs to continue to listen to, and connect with others in this journey. Rejecting his fathers way, rejecting blind obedience, mustn't mean rejecting others. For that way lies the Dean who would walk away butthurt from Sam.
Ruby, of course, is dead. Killed by Dean, which I think could be seen as symbolic. I think Sam's struggle this coming season to heal himself, and his rift from Dean, will help lead him to a better sense of who Sam Winchester is.
Everyone else already thinks he's awesome.