Jarvan IV is a strong and simple champion with a high skill ceiling but a low skill floor. You can pick him up and learn his combos within 1 game in the jungle using a standard bruiser build (goredrinker, sterak's gage, etc.)

In the top lane you run a similar bruiser build which works really well.

But in the Mid lane he is a different beast. You change your play style into an AD assassin, roaming with your jungler to pick off every person in sight. His combo turns from a simple assisting jungler's combo with a knock and cage ult into an oppressive and aggressive layers kit to an unpredictable pick.

Your job as jarvan mid is to be a front lining assassin, you dash through and pick off the backline, making plays with his ultimate and making Demacia Proud!

In the early game, stick to CSing like fizz and Zed. Level 2 marks your first power spike. Against a weaker, more timid lane, you can allow them to push up, place the demacian standard(E) behind them then joust(Q) to it, knocking them up then follow through with an ignite and auto attacks to secure an early first blood. Against a more oppressive lane, you may want to wait till level 3 to trade safely, in most trades you will lose 20% of your health while they lose 40-60%. After the trade, freeze your lane under tower, reach level 6 then repeat the knock up but cage them in with your ultimate(R) and use shield(W) to raise your attack speed and slow them. By now you should have a kill.

Base either after the kill or when you run low on mana.

The benefit of playing lethality Jarvan is his base stats. Just like lethality Garen, you are meant to build as a bruiser so you have high base health and resistances without building items. This makes Jarvan a tanky assassin that doesn't suffer as much like other AD assassins.

For mid game no matter, what, take tower, roam and keep taking kills and assist your team since you now play the role of a second jungler. His Q+E combo gives him good escape and jungle movement like any other roaming assassin like fizz and talon.

By late game, use your ultimate to make big plays like caging the jungler in a smite fight. Defend your team during any side objective siege. You don't do a crazy amount of damage so stay behind baron pit or dragon pit, distract the enemy and win for your team.

Remember, this pick wont always work but its extremely fun so take it into your games, no one will expect the amount of damage you will be able to deal and picking jarvan mid is always tough for your enemy since its rare to even see a jarvan in lower ranks, so have fun with it!