The Jets were not 4-12 last year because of their defense. They weren't a tremendous defense, but they were OK - it was the offense that was awful. In typical Jets fashion, they gave up a ton of yards, but not very many points. Let's look at the final 9 games of last year (the first 8 they did give up a lot of points):
-13-3 loss to Buffalo
-23-20 loss to Washington
-19-16 win over Pittsburgh
-Thanksgiving Day (the worst of the bunch) - 34-3 loss to Dallas
-40-13 win over Miami
-24-18 loss to Cleveland
-20-10 loss to New England (when the Patriots were supposed to score 70, remember?)
-10-6 loss to Tennessee
-13-10 win over Kansas City
That's not a bad defensive stretch, points-wise.
The Jets were 29th in the league against the run, though, and went about trying to fix that for this year. New to the defense are Kris Jenkins and Calvin Pace. I feel like most of the defensive players are guys who can play more than one position. Pace, for instance, is a linebacker who used to be a defensive end. So is linebacker Bryan Thomas. I get the impression there will be a lot of defensive looks and a lot of mixing and matching players in and out.
Kerry Rhodes and Darrelle Revis are exciting young players in the secondary. I like watching them play. And I wonder if this unit isn't finished yet - the Jets were interested in John Lynch before he went to training camp with the Patriots. Now, he's been cut, so I wonder what about him was not enticing to the Patriots, though they did say they'd consider calling him back later in the year. But if the Jets were interested before, wouldn't the fact that he spend training camp with the Patriots make him even more attractive now, if for no other reason than information? Just throwing that out there.
The big difference between the Jets this year and last year is that the offense will be putting up some points. So the defense, I guess, has more room for error. And that's assuming they're as good as last year. If they're better, the team will be improved on both ends of the ball.