So I played 2 days worth of J4 and I've concluded that he's basically crap. But don't just down vote me... change my mind.
1. He doesn't scale well no matter what build I try. Yes, he can snowball, but that's NOT the same thing as scaling. A scaling champ can AFK farm and win the game, because they scale. J4 can't afk farm successfully no matter what his item build is, because he just doesn't scale well enough. I've played hard scaling junglers.
2. His ganks seem meh. It's very easy for him to miss his Q+E combo, and if he does, they basically just walk away and he wasted his time attempting the gank. There are champs that can gank much more reliably than that, so I feel like if you plan to gank, why would you pick him for that?
3. His counter jungling is kind of meh. That's because he needs tiamat to farm decently. But there's plenty of junglers who can farm fast without any items. So they easily out farm him. So how's he supposed to counter jungle when he can't even keep up in farm with many junglers? Sure he can dash over a wall to escape, but that's a pretty weak counter jungle if the best you can do is run. He's a decent dueler, if he is full HP, but he has no innate life steal so he gets unhealthy on things like dragons and even just camps, where other junglers could be more healthy. And if he's lower on HP than the other jungler, he definitely can lose duels or simply have to run, which wastes his time. All of that just doesn't make him the best counter jungler.
4. His split pushing is kind of meh. I can't see that as a reason to pick him.
5. His teamfighting consists basically of inting himself and hoping his team can clean up afterwards. That's because he has no escape, if he uses his whole kit to initiate, which is his best engage. So he basically just dies, almost guaranteed, every teamfight. Unless of course his team is super far ahead and the enemy team has no damage. I mean, his engage is great but with no escape, he just dies. Plenty of junglers who can contribute just as much to a teamfight, but without the dying part.
So if he can't scale, and his ganks suck, and his counter jungling sucks, and his teamfighting sucks, I just can't put my finger down on any particular reason to pick J4. Maybe he count be a niche pick because of his armor shred, perhaps against a high armor target, but even then I think there's other junglers who also have armor shred who would be safer in that case.
I feel like J4 is one of those champs who only wins if your team was going to win anyway. Like, the very fact that he has to dive with no escape, kind of means that if your team isn't ahead, then he just feeds. Only reason J4 would win the game is if his allies are decent or the enemy is bad and the game was already predestined to be a win. He doesn't seem like a champ who would score a high grade on a losing game.