Well first of all this is my first guide, I'm speaking Spanish so excuse my English. in this guide will try to explain how to play with Jarva IV.

Pros:
Good Damage
Can cancel chaneling ultis (to save allys or the team)
It's really good in team fights

Cons:
Low mana.. in early game it's a problem....
Ulti wrong used can save the enemies

Red and Flat = Armor penetration, for me it's the best choise, your combo (E+Q) do a lot of damage

Seal and Glyph = Atack Speed, simple.. more AS = more dmg output.... (combined with life steal) = more time fighting

Well.. i start always whit Doran's Shield for defense and hp regen and go to line with my friend (never lane solo) the reason: i can haras 2 enemies easily in early game (E+Q) and we can quickly push the tower..... depending on the opposing team i can take Berserker's greaves (if the enemy don't have more than 1 stuner and is a melee team) or Mercury's boots if is a mage team or stuner team. It's important know how to farm!! just last hit and haras the enemy all time! ... get phage for the hp and atk (and the probability to slow)... get madred's if is a melee team or banshee's if is a caster team, finish your frozen and get stark's(get life steal first), clean the jungle with you team get blue buff(in 5 vs 5) or get dragon and Move speed(in 3 vs 3).. finsih the rest of items( at this point if you and your team play coordinated, the other team start to vote for surrender)

My team always have 2 melees and 1 range or caster (usually Jax, Ashe, Jarvan IV) , our playstyle: my friend(melee obviolsy) open the fight, i enter with my combo(E+Q) to cancel all chaneling spells (stop nunu ulti, malza ulti, galio, and do a mini stun to all - 1 sec aprox-), my other friend use hi's ulti( Ashe, Malza, kenen, twitch .....) cast W skill to increment your defense and.... time to dance!!! we can kill easily the other team, obviously focus on the "paper", if any try to escape, use ur ulti and kill! is easy because the enemies are desperate and will try to leave instead of fighting...just hit! and hit and hit!!!, you need know one thing! ... NEVERRR use your ulti near from a caster or range friend, that would be the death of your friend. And if your team is chasing, waiting for one of your friends one to reach so you to use your ulti to be 2 vs 1 in the fight

Exhaust... slow enemies, can save any ally and does fail the blows of the enemy (this is really good in team fight)

Ignite.... can kill enemies whith low hp, it's really good to stop Dr. Mundo (this hero good played is really OP), can stop Warwicks and Tryns..