Hello! This is my first build for Mobafire, but I've been a long time fan. I've had a lot of opportunity to play and test Jarvan IV out since he came out so I thought I'd share my build since it's been pretty successful for me in low ELO ranked games. It's pretty straightforward and only requires getting used to his specific mechanics. Let me know what you think!

I picked a simple rune page: armor pen marks, health per level seals, cooldown reduction glyphs and flat health quintessences. I figure building health makes sense both for early game and to help atma's later on. Seeing as how the health per level seals catch up to the flat health seals by level 5, they just make more sense to me.

The masteries are straightforward as well. They generally make more of a difference in the early game so I think it makes more sense to take advantage of his natural hardiness and go 21/9/0. Unless you're building crit, Lethality makes no sense and his magic damage is worthless so that makes Archaic Knowledge worthless as well. That pretty much takes the decision making out of it in the offensive tree. The defensive masteries are just for a little more early game survivability, but feel free to do whatever with those points.

As far as summoner spells go, I use Heal and Exhaust because I'm most comfortable with them. You can use whatever, though. For all those Heal haters, all I can say is I'm pretty good at getting kills by Heal baiting. To each his own, though.

In keeping with taking advantage of his early game hardiness, I like to get boots and some potions followed by Phage. He's extremely mana hungry early game so I get one mana potion and try to conserve unless I know I'll get a kill or at least good harassment out of it. By the time you finish Banshee's Veil it shouldn't be as much of an issue. Then build Atma's Impaler, finish Frozen Mallet and you're done with the core build. From here on I usually either work on Infinity Edge, Last Whisper or Sunfire Cape, depending on what your role in team fights has become. If you have a great tank, by all means just go for more damage. If you've somehow found yourself doing a lot of the tanking, go for Sunfire Cape or maybe even Warmog's. In the rare occasion that their team is full of hardcore AP casters, get Hex Drinker or maybe even Spirit Visage. If it's overwhelming AD, get Thornmail. You get the idea. Ideally you're building health to get more out of Atma's, but just the core build has made it worth having. Your core items have a lot of synergy with each other so try to continue with that in mind.

Jarvan's passive is AWESOME for last hitting minions. Take advantage of it. I'm gonna be honest. The scaling on his Q is terrible. I only really use it for the blink/knockup combo. It's just not worth the mana to use for damage or even armor reduction but it's essential to get early so I just put one point in it and forget about it. Demacian Standard gives you much more damage over its life with its increase in attack speed for you and your buddy, not to mention the small-but-helpful-early-on impact damage. Don't forget to use it to scout bushes if you need to. If you're ever attacking a tower, throw it down just for the attack speed bonus. In teamfights, do your best to knockup everyone you can, or at least specific squishies you're focusing. You can also use it to interrupt a channeling spell like Nunu's ult. In any team fight use your knockup combo whenever it becomes avaiable unless you think you'll need it to escape or stop a runner. This is important because your E raises your team's armor and attack speed so you want to make sure it's down as much as possible. Also, if you're in an iffy situation, as with any blink ability, try to position yourself to make a clean getaway. Just make sure to give yourself plenty of time as you have to use two abilities to use it. There will be tragic times you use E just a little too far away/early and use Q so fast afterwards that you stab at empty space. Then promptly die.

Easily the hardest part of playing Jarvan is deciding when to use your ult. Fortunately, the cooldown isn't terribly long so wasting it isn't too painful. The one thing you need to absolutely make sure of is to not get your teammates killed with it. In other words, don't trap a running friendly squishy in it or block off your melee DPS from getting to enemy squishies. Ideally, you want to trap friendly tanks/melee DPS in it with enemy squishies. His ult can be a game changer when used wisely. Try to think about how it can work with your teams' ults. Trap a group of enemies for Katarina, Fiddle, Nunu, Miss Fortune, Pantheon, etc. When ganking, I do my best to initiate with his knockup combo then W. If they try to run, then use your ult if you think it won't be a waste. If your knockup combo comes back up, use it as soon as your ult goes down. In a team fight or tower diving, only initiate with it if you've got good ranged DPS or champs who can otherwise make use of it. It's easy to get yourself and others killed if you make a mistake. Remember that in the jungle it can completely block passage in places. One more use: If you and a teammate(s) are running and probably will get killed anyway, use it to make sure only you get killed. Take one for the team. Your knockup combo works well for this as well.

In the early levels, do your best to zone and last hit, but unless you've got a lane partner who can make it happen, don't worry about even trying to get a kill until level 2 when you can use your knockup combo. Throw down a Demacian Standard around the enemy champ (trying to hit any minions you can as well) every once in a while to get a little damage and scare them away. If the time is right and your lane partner is ready, go for the knockup and then exhaust. Sometimes if mana hasn't been an issue I'll use the knockup plus a first hit to utilize his passive then run away just for harass. Again, you don't want to do this kind of stuff too much as you will be out of mana in no time. When you hit level 4 you'll have a slow and shield as well. Again, don't go too crazy with it. Another fun thing to do early game is stand at the edge of the brush and have your E selected already. As soon as someone comes close enough, give 'em the ol' 1-2 ...or E-Q.
I usually try to stay in lane until I have the 1315g for Phage. When you come back from that, things should feel much better as you do more damage, have more life and of course have full mana again. Don't be afraid to start getting more aggressive.

On a side note, if someone else is jungling, Jarvan does fine solo top as he last hits well and can survive a few hits. Once you get a few levels on the enemies in your lane, it shouldn't be hard to push them back. Early on, though, you have to play extremely conservatively and will probably have to tower hug a bunch. Just be patient.

I try to jungle if no one else on my team plans on doing it. It's always good to have a jungler and Jarvan has ideal abilities for ganking, though he's a bit slow.

Start at your blue. Try to get mid and someone else to cover the river until minions spawn to prevent an early jungle gank. When he comes up, drop an E on the golem making sure to hit his little buddies. Use smite on him and then a health pot when he starts hitting you. Focus him down first and then kill the little guys, using your E every time it comes up. When you finish with them, you'll hit level two. Now you can initiate on the wolves with your knockup combo and focus the big wolf down first, again using your E when it comes up. Don't forget to use health potions as they finish so you're healing more or less continuously. Head over to the wraiths and do it all again. Your smite will come up but don't use it yet. Next, head to the lizard camp and go for it again, making sure to use Smite again. Try to hit all three with your E-Q combo every time. Depending on how well you've done, you may or may not need to use Heal during this fight. It's not a big deal, though. Keep using your E and Q combo every time it comes up. Next hit the two golems nearby. You may get low here and you'll probably be out of potions unless you used Heal, but just make sure you go into it with around 350 health. After you kill them you should hit level 4. Head over to the wolves and do it again. You'll have your W now so don't be afraid to use that. Also, your smite will come up again so you can just kill the big wolf outright if you like. Recall after this and buy your Madred's Razors. You won't need any potions now. Go kill the wraiths again and then the two golems. You may have to wait a few seconds for them to respawn but it shouldn't be long.

At this point you have an option. You can go to the enemy jungle and take your chances, you can attempt a gank, or you can cover someone's lane if they need it. By the way, at any time if mid is getting hammered or gets killed early, don't hesitate to cover until they get back. Try to get some hits on the enemy champ, too, to make it easier for them when they get back. I usually go help top for a little while then go do a jungle run again if I'm not really needed. Usually, this is about where I stop jungling. Just be flexible. Help where it's needed and gank whenever possible. After you get Wriggle's Lantern, make sure to always utilize your free ward for the rest of the game. If laning, ward the bushes at the mouth of the river. When in doubt, ward dragon (or Baron late game). Continue your item build like normal after your jungling days are behind you. If they're extremely tanky, consider turning your Madred's razors in to Madred's Bloodrazors, but don't do this until after Atma's. Just go straight to your boots after Madred's Razors. Don't forget about smite late game. Get buffs and kill the dragon whenever possible, but remember: hard AD should get priority on red later on and casters get blue. You can also use it to steal baron; hilarity ensues.

So that's it! Jarvan isn't too hard to play, but I don't consider him a faceroll champ either. All in all, he's a lot of fun, though, and my new personal favorite. Don't expect to get crazy scores with this build, but I usually end up with a positive ktd ratio and ridiculous amounts of assists. That works for me. Basically, if your team doesn't suck, you have the power to affect a win with all your cc and especially your ult. You'll be no pushover, either. You should be able to solo most champs, but you really shine in a team fight. Just play smart.