Hello there!
I am JadedTrekkie, a man with no relevant credentials whatsoever. This is mostly a theorycraft, but I have been messing around with the build in normal games with massive successes. I hope to take it into ranked after a bit more perfection. Opgg
This is my Jarvan mid guide.

So why J4, of all champions?
J4 has a super good early game and ganking while staying relevant throughout the rest of the game. There's a reason I named this guide tanky talon with CC: He has insane burst potential, with my build of Waterwalking and Mobis early he van be all over the map while threatening to take three plates if you ever leave lane because of his E attack speed increase. He has an EQ that goes through minions, which means that it's not like Le Sin mid where they just kite you around the minion wave. He's also, as you may expect, very good at roaming and sidelaning.

Weaknesses?
Early lane phase vs things like Orianna or Syndra. Cassiopeia is an especially hard matchup but you win the 1v1 at lvl 6 provided you dont get miasma'd out of your EQ and dodge her R with yours (or turning around lol). You've very immobile and vulnerable if you use any of your ranged abilities to snag cs, leaving you open to get dove.

Playstyle:
What I try to do every game is this: since most midlaners are squishy, unless I'm against like Galio or something I'll try to get a jg at lvl 2 to gank me. Try to help the jungler on crab and drag, as well as shove for prio to rotate if they invade. With waterwalking, roaming is also super easy so you can get really free ganks off and not miss a single cs if you play it right. (Side note: to roam, you want your wave to be in one of 2 places: Under the enemy tower, aka Shove'n'Roam, or slowpushing into you BUT NOT CRASHING, so you can pick up the CS later.)

Passive - Martial Cadence
INNATE: Jarvan IV's basic attacks deal 8% of target's current health Attack damage physical damage, with a minimum of 20 damage and capped at 400 damage against non-champions. (6s CD)

Essentially you have a really fast auto that deals increased damage first time you hit something. It makes CSing a breeze and what contributes to J4's oneshot potential. You can use this to your advantage when waveclearing with tiamat to go around to every minion and auto them once, speeding up your waveclear massively.

Q - Dragon Strike
ACTIVE: Jarvan IV extends his lance in the target direction, dealing physical damage to all enemies and reducing their armor for 3 seconds.
PHYSICAL DAMAGE:
80 / 120 / 160 / 200 / 240 (+ 120% bonus AD)
ARMOR REDUCTION:
10 / 14 / 18 / 22 / 26% of target's armor
If the lance connects with a deployed Demacian Standard Demacian Standard, Jarvan IV Dash dashes to it, knocking up nearby enemies along his path for 0.75 seconds.

Poke, get cs, waveclear, cleaves armor. Not much to this, but it IS part of your dash combo so don't waste it. Max 1st.

W - Golden Aegis
ACTIVE: Jarvan IV slows all nearby enemies for 2 seconds.
SLOW:
15 / 20 / 25 / 30 / 35%
BASE SHIELD:
60 / 85 / 110 / 135 / 160
Jarvan IV also shields himself for 5 seconds, increased by 1.5% of his maximum health for each enemy champion hit by Golden Aegis.

Shield, slows, procs electrocute. Just a bigger Randuin's active with a shield tacked on. Not that good, but does help proc electrocute.

E - Demacian Stand
PASSIVE: Jarvan IV gains permanent attack speed.

BONUS ATTACK SPEED:
20 / 22.5 / 25 / 27.5 / 30%
Demacian Standard
ACTIVE: Jarvan IV throws a Demacian flag to the target location, dealing magic damage to all nearby enemies.
MAGIC DAMAGE:
80 / 120 / 160 / 200 / 240 (+ 80% AP)
The flag then remains for 8 seconds, granting sight around it and providing an aura that grants Demacian Standard's bonus attack speed to all nearby allied champions.

Ranged ability that helps take towers and other objectives like drags that can also help you get cs. It isn't written here (it's by the Q), but if you hit your flag with your Q, you dash to the flag and knockup anyone in your way for 0.75s.

R - Cataclysm
ACTIVE: Jarvan IV Dash leaps unstoppably to an enemy champion and deals physical damage to all nearby enemies upon arrival.
PHYSICAL DAMAGE:
200 / 325 / 450 (+ 150% bonus AD)
Upon impact, he creates a circle of impassable terrain that displaces units inside it, though not rendering them airborne. The terrain lasts for 3.5 seconds and grants sight around it. Cataclysm can be recast after 1 second.

RECAST: Jarvan IV destroys the terrain created by Cataclysm.

You jump on someone unstoppably and deal dmg to everyone while making a little donut in the ground where they can't leave. After 1s, you can recast to remove the wall. Your main engage tool, just jump on them and now 4 people are in a tight space with you. I'm sure they're enjoying it.