I love season 4. There’s a lot of reasons why I love it, but the biggest reason is the characters and their relationships with one another. Dean and Sam’s relationship being strained due to Sam’s demon blood addiction and Dean’s inability to open up with him about what happened in Hell, the early interactions between Dean and Cas, the few but intense interactions between Cas and Sam, Sam and Ruby’s unhealthy but compelling relationship.
I’m sure we’ve all either seen other people’s meta or come to the conclusion ourselves that Ruby and Cas are framed as parallels in season 4. Each of them has their respective relationships with one of the Winchesters, which in turn further cements the season 4/5 symbolism of Dean being associated with Heaven and Sam being associated with Hell. There’s also a strong parallel between them with Cas rescuing Dean from Hell, while Ruby was there for Sam while Dean was dead.
That being said, just because Character A is paralleled to Character B, does not mean that they can’t also be paralleled to Character C. I bring this up because there’s another parallel in season 4 that feels so obvious to me, but I don’t think I’ve ever seen meta or other discussions about it. Maybe it exists and I just missed it, idk. Regardless:
Ruby isn’t just a parallel to Cas; she’s a parallel to Dean.
Both of them are the favourites of a particular white-eyed demon. I don’t think it’s an accident that Dean apprenticed under Alastair, who described him as “having promise”, and Ruby was secretly working for Lilith the whole time, where she described herself as being “the most loyal”.
More than anything else, though, the parallel is strongest in relation to Sam and Cas. Because Ruby fed Sam her blood, got him addicted to it, dragged him down to her level. Made him more demonic, made him less human, in a way. She was the Serpent and the Forbidden Fruit all in one, tempting him away from the light and towards darkness.
Is that not exactly what Dean did with Cas? Did he not convince him to side with humanity over his own brothers and sisters? Did he not also drag him down to his level, on Earth rather than in Heaven? Isn’t a huge chunk of meta about Dean & Cas’ relationship about how Cas is more human than angel?
Compare Dean’s speech to Cas in Lucifer Rising when he breaks through the brainwashing to Ruby’s speech to Sam at the end of the same episode:
“Destiny? Don't give me that "holy" crap. Destiny, God's plan... It's all a bunch of lies, you poor, stupid son of a bitch! It's just a way for your bosses to keep me and keep you in line! You know what's real? People, families -- that's real. And you're gonna watch them all burn?[...] You can take your peace... and shove it up your lily-white ass. 'Cause I'll take the pain and the guilt. I'll even take Sam as is. It's a lot better than being some Stepford bitch in paradise. This is simple, Cas! No more crap about being a good soldier. There is a right and there is a wrong here, and you know it. Look at me! You know it! You were gonna help me once, weren't you? You were gonna warn me about all this, before they dragged you back to Bible camp. Help me -- now. Please.”
“You did it. I mean, it was a little touch-and-go there for a while, but... you did it. [...] You don't even know how hard this was! All the demons out for my head. No one knew. I was the best of those sons of bitches! The most loyal! Not even Alastair knew! Only Lilith! Yeah, I'm sure you're a little angry right now, but, I mean, come on, Sam! Even you have to admit -- I'm -- I'm awesome! [...] No. It wasn't the blood. It was you... and your choices. I just gave you the options, and you chose the right path every time. You didn't need the feather to fly, you had it in you the whole time, Dumbo! I know it's hard to see it now... but this is a miracle. So long coming. Everything Azazel did, and Lilith did. Just to get you here. And you were the only one who could do it. [...]Because... because it had to be you, Sammy. It always had to be you. You saved us. You set him free. And he's gonna be grateful. He's gonna repay you in ways that you can't even imagine.”
The only significant difference is that, since we’re human and we’re, y’know, supposed to like humanity, Dean tempting & corrupting Cas is framed as a good thing, while Ruby tempting & corrupting Sam is framed as a bad thing - again, because we’re supposed to hate demons.