Hershel's role as the moral compass of the show and our people is put to the ultimate test. We learn that he's never point-blank stabbed a walker in the head before tonight. Killing is close to being against his religion. But the loss of people he truly loves is even less acceptable. So he does what needs to be done to give Glenn every possible chance, and the combination of the killing, with Maggie's help, and his knowledge about how to keep people alive to the very end, works. With the meds that Darryl, Michonne, and Tyreese at last return with, Glenn will survive. And Sasha will too.
But the deadly flu is not the only mortal threat that our people face tonight. The gates, ever in danger of coming down as the walkers press against them in increasing numbers, do just that. Fortunately for Rick, Carl is with him, and the two manage to mow down the requisite number of walkers. It's inspiring to see Carl fighting right alongside his father, even though we all know that this kind of life is of course not whatt Rick wanted for his son. But it's the kind of life that's needed for our group to survive.
So the new plague is contained by the meds, and the wall is buttressed, and all of major characters survive, but of course no one's out of danger yet. The camera shot from beyond the compound, with someone looking at the compound, turns out to be not from Carol's point of view - who might well have doubled back - but the Governor's. We knew we hadn't seen the last of him when he survived last year's final episode. And now he's back - with who knows how many people under his command, though he may have none - and I'm wondering if Carol will yet end up on someone's side in the impending battle of The Governor vs. our group.
See also The Walking Dead 4.1: The New Plague ... The Walking Dead 4.2: The Baby and the Flu ... The Walking Dead 4.3: Death in Every Corner