We are all well aware of Sam's extensive lying in S4. One particularly nasty example is his claim in Lazarus Rising that he hasn’t been using his powers because “you didn't want me to go down that road, so I didn't go down that road. It was practically your dying wish.” It's the reference to Dean's "dying wish" that makes Sam's lie seem particularly cold to me.
But the thing is, what tends to get forgotten by people (including me), is that Sam actually started lying, or at least hiding things from Dean, in S3.
For example, the demon possessing Tammi tells him in Malleus Maleficarum that “There's a new leader rising in the West – a real leader. That's the horse to bet on, Sam, the one who's gonna tear this world apart. Thing is, this demon? It doesn't like you very much. It doesn't want the competition.” Dean doesn't find out until Jus In Bello…and he finds it out from Ruby.
Ruby did not have to push the brothers apart. They did it themselves. Sam's hiding starts even earlier in the season, when he confronts the crossroads demon behind Dean's back and against his wishes. Or maybe it starts in The Kids Are Alright when he claims to have been ordering pizza rather than researching Dean's demon deal.
As the repeated circular argument in the car (my favorite part of S3!) reveals, Sam does openly try to convince Dean to try to break the deal. Dean flatly refuses, quickly falling back upon such arguments as "because I said so" and "I am the oldest. And I'm doing what's best." In fairness, Sam also starts off their arguments angry and frustrated. But even were he an endlessly patient saint who knew exactly the right thing to say to his troubled brother, convincing Dean out of his denial would be...difficult. Going behind Dean's back really is the only option, if Sam wants to try to save his brother. And not saving his brother isn't an option. I mean, for real. Not only does he love Dean more than anything (in my humble opinion), but to have to live with Dean going to Hell for him would be horrific.
The out-of-control lying in S4 was Sam's fault. And his relationship with Jess shows that he had the temperament for lying well in place before Dean's demon deal. But the roots of Sam's S4 lying to Dean were planted at the beginning of S3...when Dean put him in a situation where all the other choices were even worse, and Sam started to realize the telling Dean the whole truth was counterproductive.