his EQ combo and auto attack passive give his engage comparable damage to that of a Leona, and your ultimate punishes ADCs hard when the flash is on CD. Discuss. If you think he is God Tier vote yes and post why! Enjoy ~~
No sustain. Not tanky without some gold behind him. He can do it. I've done it. It's a kill lane. If you get ahead you dominate lane and are terrifying. If you don't you're a poor man's leona without her ranged stun and insane lockdown potential.
Jarvin is actually a pretty good support if you know how to play him. He has poke, an AS steroid that affects him and the ADC, knockup CC, and an all in ultimate that can destroy the enemy bot lane.
To everyone saying "why not just leona, flash counters his cc, blah blah blah" Let me explain to you when you are wrong.
You aren't totally wrong, leona IS tankier naturally and she has more single target cc.
However, as the game progresses and carries get thier qss and mikaels and all that, Jarvan's Cataclysm becomes some of the most guaranteed cc in the game. Also, agaisnt things like Morgana shield and sivir's spell shield. You can't break his cc because it isn't really cc. And being able to blow 2 or more flashes with 1 ult is worth anyway.
You're also wrong when you say he has less sustain than Leona. You've clearly never got behind as leona, because she is WORTHLESS if you can't win a fight. Jarvan can still sit back and poke with his q, he can bush check with his e, and he can trade in short bursts with his w. And the best part, he can proc his targons at range with q or e without risking diving in and dying or having to face tank the wave.
So, I don't think he's nessecarilly going to replace your Jannas and your Threshes, but he's a very good counter pick to a lot of things. VERY good.
Oh, also in response to the notion that Jarvan has to dive in to do his stuff, well, yes, technically that is true, but unlike leona, he doesn't have to STAY in. You are allowed to e/q OUT of your own cataclysm as well as to close the gap to get it down. leona doesn't have that option.
Really, to my mind anyway, Jarvan is actually the safer, less Ham option between the two, with the added bonus for soloq that he can actually do things on his own without his team being there in order to make his damage worth something.
Oh, and the last advantage he has over basically every other support in the game is his ability to get deep into the jungle and put down wards. Even Lee Sin can't get wards where Jarvan can because lee needs to keep some for his escape. Not Jarvan. You have an insanely dangerous place you need a ward down? J4's your man to sneak it in there.
Viable? close enough. You could do a lot worse than J4 support, he's got very similar strengths to Leona, and has a few edges she doesn't have (while she's got a few things he can't do).
Big issue with J4 is end-game though. His HP shield is certainly strong enough in-lane, but come late game? If you don't get a solid start or are coming from behind, J4's shield strikes me as noooooot quite tanky enough. 510 HP is quite solid, but if Leona's armor/mr steroid offers considerably better protection at higher levels off her base stats alone, and scales quite well with any tank items she can afford. Also - if Leona is behind, she can still AoE CC an enemy team while keeping back a reasonably safe distance, J4 "must" dive in to apply his kit's strength.
If you do well - he's a great pick. But good supports aren't about being good when they get a lead. A big part of a successful support champion is being a serious threat when they're very far behind.
As for God Tier?
Janna is god-tier.
Not many champions come close to how good Janna can support.
J4 is not one of those champions.
Really would not call him god-tier.
I mained j4 supp for like most of s4 and early s5. still my top 3 supps, just like to mix it up a lot.
He can actually play a lot like Leona. He has less CC but gains the ability to poke without engaging, and the armor shred on his Q can be nasty in combination with an ADC following up. His shield is good for trades, and after 6 you can slaughter anyone who overextends and doesn't have an escape. Take relic shield for sustain.
Try comboing with gnar or vayne for knockback stuns into your ult's temporary terrain. Or MF so she can shred enemies while you trap them.
Can he go bot? Yeah, full ad jarvan LOVES having two squishy targets to wreck.
But he is not a support. He is a melee ADC if he is botlane.
I'm sure he's viable, but I could see him having problems getting poked out of lane, where at least leona has a massive armor/MR steroid, he also scales very well with damage or by going bruiser and his cc isn't really comparable to that of most supports (being a 1 second knockup that feels like it's the size of a mundo cleaver now as well as an ult that is countered by the most common summoner spell in the game, unless you can bait it of course?) That being said, at that point why not just take Leona? Her CC is superior and she is naturally more durable, her damage is eh but I feel like she has a lot more utility.
Not hating on J4, I absolutely love that champ, just stating that if I could choose between a leona or him, gonna go with leona everytime. A lot of champs are "viable" as supports but not necessarily great (glaring at you VK and Lux.)
He is viable bottom lane. But is a risky pick.
Same as every "Kill Lane" Since he isn't normally a support, he falls to pieces without a decent gold income. Meaning if you don't secure a HUGE lead early on, you will be near useless late game since you won't have the durability to survive before even getting your ult off.
But if you do get that lead, its a nightmare for the enemy team to deal with.
I would NOT pick Jarvan IV support against heavy poke lanes. 
literally wrecks him early on if she is picked to counter you.
