Most appealing is Cullen's devotion to his new wife and baby. Cullen's even sleeping with Naomi in season 3 was a breakthrough for the character, who was much more circumspect - and boring, in this regard - in previous seasons. We could only assume that Cullen felt some kind of deep attraction to the Mormon farmer's daughter, and, if that was the case, it was satisfying and not surprising to see Cullen not only standing by but enjoying his new family in episodes 4.1-2. (There's a new actress playing Naomi, but that's fine.)
Cullin's confrontation with the Swede ("I'm Norwegian!"), masquerading as the bishop he killed, was also satisfying. As cannily demented and dangerous as the Swede is, he has never been a match for Cullen and his intelligence face to face.
The other big news in season 4 is Ulysses S. Grant's appointment of Campbell as provisional governor of Wyoming, the current forward point of the railroad. Campbell is well-played by Jake Weber (last seen to good effect as Joe on Medium, and to not such good effect as a lunatic cult-leader on The Following), and the character will provide a good opponent to Durant, and likely Cullen as well, before this season is over. Indeed, Durant will need Cullen to stand up to Campbell and his well-dressed men, the cutting violent edge of civilization.
One looming question is what happened to Elam, last seen at the end of season 3 on the wrong end of a bear. We learn that his horse came back but not Elam, and since Common's name is not in the credits, it's not unreasonable to conclude that the bear consumed him. However, I think Elam is far too important and powerful a character leave the series this way, or in any way at this point. I'm looking forward to seeing Cullen go out and find him.
And I'm looking forward to this season of Hell on Wheels in any case. The series continues to get better, was good in the first place, and is a pleasure to see.
See also Hell on Wheels 3.1-2: Bohannan in Command ... Hell on Wheels 3.3: Talking and Walking ... Hell on Wheels 3.4: Extreme Lacrosse ... Hell on Wheels 3.5: The Glove ... Hell on Wheels 3.6: The Man in Charge ...Hell on Wheels 3.7: Water, Water ... Hell on Wheels 3.8: Canterbury Tales ...Hell on Wheels 3.9: Shoot-Out and Truths ... Hell on Wheels Season 3 finale: Train Calling in the Distance
And see also Hell on Wheels: Blood, Sweat, and Tears on the Track, and the Telegraph ... Hell on Wheels 1.6: Horse vs. Rail ... Hell on Wheels 1.8: Multiple Tracks ... Hell on Wheels 1.9: Historical Inevitable and Unknown ... Hell on Wheels Season One Finale: Greek Tragedy, Western Style