Hi, my aKa is Alexcamill and i play on EU servers since 2 years, this is my first guide on Mobafire, I saw a lot a missunderstood builds (imo) of Jarvan IV so i decided to show you how i play him, even if my English is low... I apologize!

First of all this guide is about a hybrid DPS/Tanky Jarvan IV, i mean you won't carry the game with this build, but you will bring an advantage to your team if you do it the right way.
Playing Jarvan IV as a Carry is, in my opinion, a big waste, i saw a lot of Jarvan buying Madred/BLack Cleaver/Infinite Edge, on the paper it seem find but in practice it's terribly bad.
When they build him this way they are focused and taked down in less than 5-10sec during team fights, so the damage output is lower than a AS Janna. "lolubad!"

I'm not saying this guide is perfect, but it work pretty good for me, look:

http://hpics.li/4193696 OR this one today http://hpics.li/9f8a318

Rofl total domination see?

K/D/A Ratios are not the best but i'am not pretending pentakill, just a good teambuild who work amazingly well.
This is not pubstomp, i played premade with my mate Amadouce (playing eve/Morgana) and Moagault(Xin/Amumu).
I hope it work great for you too!

Plz rate and comment!

Pros

.Strong Champ with amazing Passive
.Can harass opponent easily early game
.Good escape mechanism with his (E) + (Q) combo + Flash
.His ult is devastating when well timed during team fights
.Easy last hit with Passive + (Q)
.scale pretty well late game

Cons

.His ult can wipe your team when bad timed during team fights
.Mana starved until Manamune
.can be useless if bad build order (like lack of DPS or survivability at keys moments)

Ok, armor pen is what you need for farm + harass with your (Q) ability Dragon Strike.

Flat Mana per 5 sec/Mana per 5sec/lvl Seal allow you to never run out of mana in order to not only harass with (Q)Dragon Strike but also flee if ganked by jungler with your (E)Demacian Standard (Q)Dragon Strike combo.

Magic resist Glyph in order to shift you into a strong laner, with doran regen and this MR you can take enemies will see that harass you is useless, like shen with his Vorpal Blade.

PS: you can try 1 Flat hp quint + 2 Movement speed quint instead of 2x flat hp + flat MR

I go for a straigt 21 Defensive, going for Tenacity. Strenght of Spirit will allow you to stay in lane longer, Veteran Scars and Ardor explain by themselves :)

PS: Evasion and Nimbleness are IMO facultative, they are usefull when you go for Ninja Tabi against a Strong AD team, but even in that case it's often smarter to go Mercury's Tread (say Freddy... --> [-]).

Lefting points go to Offensiv branch, Deadliness because you are not AP, improved Exhaust Cripple cause Exhaust = best summoner spell in this game, and CDR + ArPen explain by themselves too.

Ok, Jarvan have naturally one escape machanism, his combo Demacian Standard + Dragon Strike who allow him to escape or chase on great distances, but now add a Flash, and even a kassadin can't deal with you. Ok flash cooldown is fat, but save it for apropriate times.
Even for Engaging Flash can be usefull, Flash + Cataclysm and that's it, only if you know that your team mates won't let you down, don't do that in pub games.

Exhaust is, in my opinion, one of the greatest sumoners skill in this game, since his recent change i find him even more usefull, even against magics damage dealer.
With Exhaust ON you will mostly win your 1v1 fights.

Ok, to max first :

-(Q)Dragon Strike, this is your bread and butter as a Jarvan IV, or V.

-(E) Demacian Standard, this is you second great skill, with amazing passiv early game, ans allow you to bump people in the air, or just farm creep waves easily.(don't waste your mana with it before lvl 6-7, it's using a big peace of mana)

-(W) Golden Aegis Another great skill, but you will never use the passive activation, believe me, just use it to chase cutties who try to escape, the slow is insane during team fight and will assure you and your team MovSpeed domination, that's great.
You can use it to deflect a spell early but care at your low mana pool.

-(R) Cataclysm (devs play WoW) This skill is a very versatil one, it can turn the tide if you use it find, but i mostly use it when i'am chasing or at the middle of teamfights to save a chased carry.
It's a risky move to engage with it, do it on very special conditions: your team is near of you and aware, you are not under an enemie turret, the opponent didn't have any Flash like hability.

Core items are Manamune, Trinity and Banshee's veil.

Manamune cause this champ is very mana dependant during teamfights, plus the 20 + 2% of actual mana bonus damages, this is awsome.

Your cooldowns are low, so Trinity 150% next attack is very usefull, slow proc is very nice too.

Banshee's Veil cause every mana champions should have this late game.

Ok now, thoses items are kinda situational too, Hexdrinker is good against magic buster team, but if your team have no tank, better to get a Warmogs early for exemple.

For the last item i think you should choose something who is adapted with the late game you have, if you are owning, and offtanking, go for one of the BF Sword's recipes (Black Cleaver if their is hard tank, Bloothirther if you need moar survivability).

At end game you will reach 100 armor with your passive of Demacian Standard, so if you need be more tanky, you can buy a Randuin's Omen or something like that, but usually it's k for armor.

Jarvan has some great innitiating skills, during team fight throw you (E)Demacian Standard and then (Q)Dragon Strike to bump ennemies then pop you slow (W)Golden Aegis. Great move because armor reduced by 10-20% for 3sec is pretty effective against all kind of opponents, slow allow your team to reach easily their weaker champs and your Demacian Standard will buff you and your team +AS, +armor.
(Special you cause people think you have or the passive OR the active, you have it twice when activated)

This combo is pretty effective against low elo players who cross the Demacian Standard line, you know you can bump them this way.

Use your demacian standard to make them PHEAR, explaination:

put it betwin their team and your team before teamfights, if they cross the line you just have to Dragon Strike and it goes on.

Ok, Jarvan IV is good at solo laning, even midding, but great a solo laning if their is a jungler.
Jarvan IV's passive Martial Cadence is GREAT, this allow you to last hit every things, and if you are dominated by 2 peski ranges like Ashe/???? you can last hit with your (Q)Dragon Strike.
When solo laning you can ealisy harass opponents and get lane domination, with mana reg runes you are able to last hit + harass opponents, it's pretty good!