Jarvan IV's flag applies a constant attack speed buff to you and your teammates. When you set the flag down, Aery will fly to the nearest teammate and shield them. Aery will actually constantly refresh onto the nearest ally as long as they're in range of your flag (which has a massive radius btw).

Don't be fooled by Jarvan's typical bruiser carry potential that you see from him in top lane or jungle, J4 has insane buffing capabilities and you can easily carry and protect your entire team with his kit.

The main thing to focus on J4 is just keeping your E up so you can buff your carry. Use your E to provide your entire team ridiculous attack speed bonuses in teamfights, and use your Q to completely demolish an opponent's armor. If an enemy ever takes one wrong step, Jarvan is there to perma cc and debilitate them until they're bursted down.

I'll be totally honest, early laning is very rough for Jarvan IV. You actually have a pretty nice advantage level 1 and 2. Starting E allows you to get really nice poke off and a little bit of shielding for your ADC to last hit even in really volatile matchups.

Level 2, you get Q, so you can use your flag n drag combo. I would not recommend using this agressively as soon as you get it unless you know you have a heavy advantage. I would recommend saving your flag n drag for the enemy to get cocky and try to go in for a trade. You can easily do a lot of damage to them by knocking them up when they try to go in for damage.

Levels 3-6, just focus on playing defensively and not taking too much damage. If you're getting a gank you can use Jarvan's E-Q-W combo to completely cc your opponent while your jungler comes in. The enemy rarely expects you to turn around and all in with Jarvan like that, and the shield you get from your W at low levels actually does a lot to help you survive.

Jarvan actually benefits heavily from roaming, I wouldnt recommend sticking to your ADC the whole game or at least after tower plating falls. At this point you can gank mid pretty easily and make sure to get your other laners ahead. His E actually really helps with taking towers because of the group attack speed bonus.

You may be a support but always ALWAYS focus on taking objectives on Jarvan IV. Your close proximity to the dragon should help you a ton with securing it every time. Don't be afraid to play bodyguard for your jungler while your lane has heavy prio or is crashing into their tower. Roaming all the way up to the Rift Herald is a nice play as well. You can easily solo it if you really think you're in a safe position to do so as well.

Late game Jarvan IV has it a little rough, you're not really tanky enough to be a heavy frontliner unless you're just swimming in gold and you have a Knight's Vow or Warmog's. At this point you just need to follow any bruiser or heavy damage teammate you have and use your E and items to buff them while using your Q whenever you can to reduce the enemies' armor.

If you think it's safe to do so and you can burst them down in time, then flag n dragging and then using your W to slow an enemy is basically an instadeath combo against most champs. Your passive also allows for some pretty nice burst, though I wouldn't really recommend sitting on top of them too much and autoattacking most of the time.

Taking the tanky bruiser aspect out of Jarvan IV has pretty adverse effects on his ult. A lot of the time you'll find that using it as an engage tool is basically just trapping you more than anything, and you'll likely die if you use it that way. You really just want to use it as a chasedown tool. If the enemy doesn't have a way to escape, then it is literally the LONGEST movement impairing ability in the game.

If you feel like you're going to die if you spend too long trapped in your crater, you can actually just flag n drag out of it for a nice knockup while also keeping your targets trapped.

If you're being chased by an enemy, you can also ult on top of them to trap them and then flag n drag out to try to escape as well. At the very least your teammates are totally safe by then and maybe you die, but that can still have a really big impact. Don't be afraid to suicide in to save your whole team ^w^