Pete Campbell (Vincent Kartheiser) began the show as an antagonist for Don, who during the first season would attempt unsuccessfully to blackmail him about his past in order to climb the corporate ladder. Over time we’ve seen that Pete can be as repulsive as he is tragic. This season had a healthy dose of both as Pete reluctantly settled into life in the suburbs -- keep in mind this is a character who had said he'd rather die in Manhattan than flee for safer territories -- and became restless even as he achieved great success in his career. He's finally in a position to get almost everything he wants at work, outpacing Roger and Ken for accounts, but still finds himself unfulfilled. Ironically, this season Pete and Don seem to have switched places, with Pete looking to fill the void with extramarital affairs.
While Pete spilling his feelings to Beth may have been a little too on-the-nose dialogue-wise, Kartheiser was still strong. In fact, Pete was responsible for many highlights this season including several fistfights, his night with a prostitute and the Pete-centric "Signal 30," which was an early season standout diving headfirst into his psyche (drip, drip). With all the death imagery hanging around this season, many had pegged Pete to finally put that office shotgun to use and off himself but he appears safe for now -- that fate fell, unfortunately, to poor Lane.
Lane Pryce (Jared Harris) was a character that always seemed to get the shit end of the stick. One of his only moments of triumph came in the Season 3 finale when he told off his former employers with a jolly "Happy Christmas" before claiming his partnership at the newly formed company that bears his name. But barring that incident, he's been beaten down (literally) by his father, disrespected at work (though he did claim a boxing victory over Pete) and hounded by the taxman. Had he not been quite so proud he might've approached his partners for a loan but as an upper crust British gentleman he made the mistake of "borrowing" some funds from the company early on in the season, which he never had a chance to repay. Many anticipated this development would pay off dramatically down the line but few probably expected it to resolve quite as darkly as it did. After Don approaches Lane for embezzling funds from the company and offers him the weekend to resign, covering off on all debts, Lane decides that rather than explain this to his family, he'll take his own life.
After a failed attempt to commit the act in his new Jaguar, he stages a second try at work, and this time he doesn't miss. The partners find Lane the following morning hanging in his office accompanied by a boiler-plate resignation letter. Though many viewers may have sided with Don for doing every decent thing to conceal Lane's misdeeds and save him the humiliation (not to mention jail time), Lane obviously felt he had still been wronged and left his own body as a giant “fuck you” to Don. Harris agreed, telling Vulture, “It was a vindictive thing to do, to kill himself in the office, so without question he went back in there and wanted to be passive aggressive — the act of killing himself in the office is the aggressive part and the passive side of it is to leave a suicide note that explains nothing.”
Betty Francis (January Jones) once again received fairly minimal screentime this year, and though some may place the blame on Jones' real life pregnancy, most viewers know that Betty hasn't figured heavily into the show since she left Don in Season 3. She probably received about as much screentime this season as she did in Season 4. Though her weight gain proved an interesting development for her character, its execution was lacking, and the only real misstep of the season was the Betty-centric second episode "The Tea Leaves" (though it did spawn the brilliant parody "Fat Betty" for which we are all grateful). As the season wore on she did get some great notes to play. Betty catching the sight of young, thin Megan changing in her room was a crushing moment for her character. Throughout the season, fans experienced emotional whiplash feeling sympathy for her one moment and being reminded why she can be so unlikable the next. (And repeat.)