It's not quite the new Donna/Tig/Clay situation yet, but I'd say they took it off the backburner when it became the big season three cliffhanger. Clay and Gemma's possible involvement in Teller's death was only hinted at before and was brought up a total of two, maybe three times in the previous seasons. But now a few episodes into season four and already we have Clay looking rather worried, Gemma looking even more worried and going through Tara's shit behind her back, hidden police reports, and Unser knowing something about what happened. They're building to something, there has to be some kind of movement on that plot this year.
I didn't say it was still on the backburner, though, I said it had been there too long to feel as dire as the previous bombshells club has tried to hide. One of the chief strengths of the show has always been how things start tense and quickly ratchet upward and upward from there. Season 2, the undisputed highlight of the show, began with a very loose lid on the Donna situation and immediately threw a gang rape on top. Even last season, uneven as it was, started with a bang. If this were another show, starting things this slow (mass murdering villainous extras can't compete with shit happening to characters we're actually invested in) wouldn't even be worth mentioning, but this is a show that always hit the ground running. Killing a mole seems like it should immediately take things in a bad direction, but the immediate reaction from the feds is "we'll sit back and see what happens for a few weeks", and its not like the guys haven't skated on murdering two ATF agents before with practically no fuss. Getting in bed with the cartel is an ominous move, but so far they're getting the guys out of jams.
I actually rather enjoyed both these episodes, but they represent a change of pace for the show. I just hope it's due to a deliberate decision to switch things up and not simply an inability to match what's come before.