Flash- flash gives you an escape and gives you an extra gap closer in those situations when you are just out of range, or when you need to get away from that one fed carry. Flash’s viability makes it an undeniable choice for J4

Ignite- I find ignite to be very useful on J4, especially when you want to go aggressive in the early game. If you get level two before your opponent, you can engage and go for kill before they get level 2. Remember, your passive gives you extra damage on your first auto against an enemy each 7 seconds. This should be your bread and butter engage: use your e-q combo to knock them up, then follow up with your empowered auto and ignite.

Teleport- Teleport, or TP gives J4 lots of map pressure, and away to comfortably back and come back too lane. I recommend taking TP when up against someone that can beat you in the early game like: Darius and Renekton. Also if your botlane is very passive (Vayne and Soraka botlane)
And gives you an opening to apply and relieve pressure for your botlane, this will make it extremely easy for them to late game, in which your team shines the most.

Rank 1
Fury is my choice because the extra attack speed will get you an extra auto.

Sorcery is not as favorable on J4

Rank 2
Double edged sword is really good on jarvan. The 3% extra damage can have a big effect on the entire game.

Feast can be a second option if you find yourself not having enough burst, you can take bad trades, and heal up with the burst.

Expose weakness can be very viable if you have an all-in team comp (amumu, malphite, or fiddlesticks)

Rank 3
VamprismVampirism can be very useful in trades that are very long. This is good against a tank where you're gonna be draining them for huge amounts. When paired with Death’s dance, this can be a deadly mastery.

Natural talent is not as viable as vampirism because it's not as powerful

Rank 4
Bounty hunter is a risk reward type of mastery. If you are confident you will get fed then bounty hunter can show true value.

If you want a mastery you can rely on, then oppressor will show more value than bounty hunter. This mastery is extremely reliable and synergizes with your EQ combo knock up, and your slow from w and ult.

Rank 1
Wanderer is a great mastery if you find yourself roaming a lot, also since you don’t have TP, the extra movespeed helps you get back to lane if you need to quickly back.

Savagery is great for junglers, but for laners, it's not that good. Wanderer completely out values and out scales savagery.

Rank 2
Runic affinity is something great on junglers and mana hungry, but on laners like jarvan that won’t get a buff that often, runic affinity does not show good value.

When you find yourself against a counter (renekton/darius) secret stash can give you the sustain that you need. The empowered potions can mean the difference between life or death, a must have against counters.

Assassin is my personal favorite, and it only adds to the insane damage that you have, this gives your damage a heavy boost and should be an auto include against squishies.

Rank 3
MediationIf you tend to have mana problems, meditation can help you stay in lane with a healthy amount of mana without having to back.

On the other hand, if you can manage your mana, merciless is just another factor that boosts your damage. Yet instead of a flat boost, it has a condition. It helps your damage against enemies with 40% health or lower. This helps you finish off a target when either assassinating a squishy or getting first blood. When comboed with your passive, you can chunk an enemy with one auto to secure either first blood or a regular kill.

Rank 4
Dangerous game is a sweet mastery because if you get chunked while assassinating an enemy, you’ll get a chunk of your health and mana back, helping yous stay in the fight for much longer

Bandit is an ok mastery, but i find it much more useful on someone who auto-attacks more often like Irelia.

Rank 5
Thunder-lord's decree is a great mastery on J4, because it will proc right after your EQ combo, and will add a significant amount of damage to your burst.

Greater Mark of Armor PenetrationArmor penetration- These are my personal favorite on J4 because since you will usually get an early kill the enemy will build armor. These make your life much easier especially when paired with maw and yommu’s.

Attack Damage- I don’t recommend these because of all the ad you will be building, I mean, it’s the FULL AD JARVAN you won’t need any extra. Personally, armor penetration just feels better but if you want you can take the attack damage

Flat Armor- Most of the time, you will be against an AD top, and the extra armor never hurts. This can really come in handy and will make you slightly more tanky.

Flat Health- When against an AP top lane, or mostly AP team, you gonna want to take these seals. The straight health boost against the AP laners and Teams really make a difference.

Scaling Health and Armor- I personally don’t like these compared to the up-front health and armor, just don’t take them.

Flat Magic Resist- I like these because they can make you a little bit tankier, rather than the Cooldown reduction, i would personally take these.

Flat Cooldown Reduction- Against squishy match ups because you can bully them out of lane and not get really punished for it. It’s a big risk, but a huge reward.

Attack Damage- Since you already have the armor penetration marks the Attack Damage quints will really help you get that early kill. These can be very useful in the early to mid game.

Long Sword:

] Do to Jarvan's strong early game, starting long sword is a perfect fit. The long sword gives you extra early damage, and Builds into all of your first back items.

Potions

: The three health potions gives you sustain if you take a bad trade, and gives the health to go for a risky kill. If you play against a renekton, and he has ignite and starts doran's blade, if you both ignite each other, and are both on the verge of death, you will have more potions, there fore giving you the upper hand.

Corrupting Potion:

If you're against someone like Darius, take corrupting so you can sustain your lane and back often. The potions refill every time your return to base, so make sure to take advantage of your teleport.

Tiamat:

Unless your're playing against one of your counters, you should have enough gold (from cs dominance in lane), to buy your first back item. If your ahead, go for tiamat, the auto effect will help you push in the waves and give you time to roam. The she power it brings to your kit is truly insane.

Serrated Dirk:

If you're even, you wont have as much gold as if you were ahead, but not to worry, serrated dirk is still a very solid item on J4, giving him good armor penetration for good trades.

Phage:

If you're behind, try building a phage, it gives you some tankieness, which helps you survive against those high damage carries. Don't forget to buy pink wards and pots!

Caulfield's Warhammer

: No matter if your behind, ahead, or even, your second back should always be Cualfield's warhammer. The hammer builds into to Yoummu's and Black Cleaver, which are both key items in Jarvan's kit.