In my opinion, i think J4 is a really good early game jungler. You can easily put in early ganks at 2. Also depending on their jungler you should be able to get a smiteless blue then steal their red. Jarvan is a very flexible jungler and can help his team in many ways.

You should usually start off with the buff that is on the bot side. That way you can get a quick smiteless buff. Right after that rush your other buff. You should have smite up if you got a smiteless, so it will be easy to take the other buff. Usually after that put it in an early gank top. If your top is pushing gank mid.

You should usually on play J4 if your team comp agrees with. It usually goes great with aoe ults/skills. You have to communicate with your team before you ult or else your team wont follow up as quickly as they should.

In teamfights you should try to ult the carries then ping them so your team knows they're trapped. After you killed them the teamfight is in your favor. Just knock up the people that are left with your E to Q combo. After you win the teamfight you should look at your options and either try to push a lane, get baron, or get dragon(probably your last option. Also it will take time for you to know when to ult but when you start using it you will know you should and when you shouldn't.

Pros
*Great cc
*Tanky
*Great peel
*Great fight
initiation

Cons
*Mana hungry
*Falls off
hard late game

Your runes should be mainly tanky, besides your quints and marks. It is your choice if you want to get flat armor pen marks, flat ad marks, or a mix. I think armor pen is important know because of the item buffs they have been making.