I think jin is too weak

I think this post is too weak.

Well I think he's fine, so?

Jhin does lots of damage but not in the same cadence as a normal ADC.

I think you may be missing how his passive scales his damage. With his passive, his AD will end up much higher that a standard ADC would get to balance out his fixed attack speed. Also, do not underestimate the value of the missing health component of his 4th shots.

The execute damage even works on dragon and baron. In my last Jhin game, I got a 665 damage crit on a half-ish health dragon (5660 total hp) with just a BF sword + crit cloak (149 AD). The next dragon (6kish health) I had an IEdge with a total ad of 222 and got a 675 on the first clip and a 1k crit to execute it.

If the ult is too weak as you say, and I don't think it is, then buffing the per shot damage would probably be better than adding more shots. This would be both for thematic reasons (Jhin is fixated on the number 4.) and because it increases the damage without increasing the effort.

Honestly I find Jhin just fine. He doesn't need high attack speed because his individual shots make up for it. My very 1st Jhin match was mid lane vs a Syndra. I won the lane (and the game itself) because I played him smart. I didn't melee-form it with him because that's not his style. I didn't try to chase because that's not what he does. I watched my positioning, I watched the map, I learned his reload timing so I could get more CS (was weird at 1st but I picked it up after a few waves), and used his range to my advantage. One time, about level 7 or 8 my jungler invaded their blue buff (we were red team) and Syndra went after him to kill him (I don't remember who the jungler was, but he was low and Syndra was about to catch him). So I used my ult to keep Syndra slowed and assisted my team using his kit. I kept picking people off with my W for my team, I kept defending and killing with my ult, I used my traps for escapes or choke points, my Q was great for wave clear and to get multi-hits during fights.

Jhin himself is fine as long as you play Jhin his way and not another ADC's way. Focus on your damage, and don't read to much into his passive's AD boosters (the attack speed and crit). It was mathematically shown that still buying the high AD items was best build for damage output on Jhin. I build Essence Reaver, Swiftness Boots (usually, but sometimes Berserkers), Rapidfire Cannon, Bloodthirster, Infinity Edge, and the last item depends on the match itself but the other 5 are the for sure items. ER keeps your mana high due to always getting proct every 4th shot (and every normal shot that crits) and gives a lot of CDR that Jhin likes. Swiftness helps with your positioning and kiting (due to ranged champions having lower movement speed than melee champions and he wants to stay at range). Rapidfire Cannon helps his tower killing, champion killing, and his range helping him deal more damage and be more useful. Bloodthirster gives a good chunk of HP with each auto because of how much damage a single shot will give you (LS isn't as effective on him as a high attack speed champion, but that healing spike could make or break a fight). Infinity Edge gives more crit making your basic attacks crit more often with ER and RFC while increasing their damage, standard IE stuff. The last item (and note that ER should always be 1st item, boots or Rapidfire afterwards, then from there it's up to you after the 3rd item) always depends on the match. If you feel like you're dying a lot buy Guardian Angel, if they have a good portion of tank get one of the Last Whisper final items (again depending on their comp, but the higher HP damage one is usually what you want unless they have a Mundo or Darius).

For masteries I use 18-12-0, Ferocity : Fury (gives more AD to Jhin due to his passive), Feast (the healing is nice, but you can Double-Edged if you wanted), Natural Talent (2% LS from Vampirism isn't really worth), Oppressor is safer due to your W snare and your E slow so I use (but again you can Bounty Hunter if you feel like it), Battering Blows is obvious because you're AD, and for your main Keystone Deathfire Touch. It took me a little while to figure out which keystone would be best for him, this tended to be the best one. The damage scaling you get on it is amazing due to Jhin's massive AD count from his passive and goes up with Natural Talent giving you 15 AP (not a lot but it helps) to the point that each spell you damage with can deal over 400 extra damage due to your massive AD scaling you end up with. I don't remember the exact number but it is super high.

Cunning : Wanderer (obviously for helping you get to where you need to go, that 3% mixed with Swiftness and the movement speed from Rapidfire will make you speedy), Secret Stash (pretty standard and you should almost never be alone so Assassin isn't very useful and you won't always have buffs so Runic Affinity might never get used), I use Meditation to help keep my mana up until I get my ER (and honestly 1% from Merciless isn't really worth even if you had Double-Edged Sword, % damage boost isn't nearly as good as % max health damage so getting them confused is bad), and finally Dangerous Game because healing is always nice when you kill things.

Hope this helps you understand the power of Jhin and how to properly use him in matches.