J4 is a really strong early to mid game jungler. Although he falls of in the late he is still really tanky and can effectively support an AoE comp or even zone the enemy-carries by himself.

Your early game role is more like a roaming mid lane, beacause he is not tanky yet and has a lot of dmg potential. You should always starter in your bottom jungle no matter what. By doing so you are able to fast gank mid or top and you are able to counter-gank the other jungler, but to do so you have to go from your starting-buff immidiatly to your open buff to prevent counter jungleing always pull the buff in the bush. Then your first goal is to gank one of the sololanes the easiest way to do so is looking at their items to see if they have wards, keep it in mind all game long. You should variet your gank style and path to make it harder for enemies to prodict and easier to pick up kills exspecially in the lanes without flash, in that situation you should think about timing their flashes and buffs so you know when to counter jungle to make the biggesst use of it. If you see the enemy jungler on the map try to get an advantage to him by stealing stuff or ganking yourself without the danger to get counter ganked. And you have an easy target for your ult if flash is not ready for them.

A tanky J4 has one of the easiest engaging - tools in the game, its often hrad to land a great ult on all carries so you should try to get one high dmg enemy and use yourself as a wall.