when against yasuo, riven can kill him in two seconds without items at lvl 4....i understand counters, but i feel like this is a bit ridiculous.
Different champions have different power spikes. Some are early game champions, some are late game champions, some are mid-game champions.
I mean: counter-jungling a 
is not smart.
Playing a late-game champion also comes with the risk where an early game champion is going to make you miserable, barring some balancing oversight on Riot's part where they allow a late-game hypercarry like Tristana or Kog'Maw a solid lanning phase.
Also - different champion mechanics cater to different lanning phases. Yasou's mobility gives him an edge vs. skillshots, his shield vs. weak poke, and his wind wall vs. projectiles. He's pretty solid vs. a ranged skillshot-reliant champion in lane (mid-lane).
None of his defensive mechanics are particularly good vs. a committed melee champion though, as his wall is largely useless, forward mobility only places you where an opponent wants you to be, and a weak HP shield that's solid vs. poke is not going to be particularly useful vs. a lot of burst and a drawn-out ham fest.
My point here is playing Yasou into Riven is a lot like playing Tryndamere into Malphite. That's just a really unwise lane matchup.
no items? just A longsword and its Two seconds. thats a bit Extreme...
Longsword is an item.
It's not not an item.
Early game small items can make a large difference, and runes and masteries are, for all intentes and purposes, bonus items you can get early.
Not every champion is set up to hit like a kitten for the first 20 minutes of the game, some start off with their kits working as though they actually have a sword rather than a pointy stick.
And a shield that scales with ad is just the definition of bruisers that have no tank items and are tanky.
Ehhh.
Most light fighters have a defensive mechanic capable of absorbing considerably more damage than Riven's shield. 100% damage block potential is found on:
Wukong's clone can eat a champion-only ultimate or bait out something like a Tibber's Stun.
Yasou's Wind Wall can eat a Nami wave along with most mage/marksman ultimates.
Riven's HP shield doesn't have much on those mechanics, making it kind of wierd in regards to Yasou especially, seeing as his Wind Wall is one of the better of such skills. Vs. a 
team, proper timing could allow you to block all 5 of their ultimates for not only you but your entire team.
Also - Fighters in general are supposed to be tanky DPS. Dealing damage while being tanky is their shtick, regardless of what they do or do not build.
Tanky DPS that build tank like 
obviously get plenty of damage for free to fullfill the DPS part, tanky DPS champions like get a lot of defense for free so they can be tanky DPS with glass-cannon, champions like get defense that scales with her offense and defense that's only as good as her offense.
A Riven that's behind isn't exactly Yasou status tanky where she has the tools to survive long enough but not the damage, a Riven that's behind is just not tanky enough to work and not damaging enough to be much of a threat if she manages to last long enough.
please can you explain to me Riot the current thought on riven? I dont like getting two shotted at lvl 4. I want an explanation please
I'm not sure what your definition of 2-shot is, but mine would be eating 2 shots and dying.
Considering Riven's rank 1 broken wing hits do less damage than her auto-attacks, her rank 5 wing hits do less damage than an auto-attack with 225 total AD (~115 bonus), and champions tend to start with 500 HP: you're idea of 2-shot clearly isn't eating just 2 hits.
So I can only assume you mean the OHKO logic of 1 combo being a hit, in which case: the base QWER damage of pretty much anyone with burst is often incredibly close to 50% of everyone's base HP at that level. Level 18 QWER from a typical mage or assassin or tank is 1000, average level 18 HP is 2000. And if a typical AP gets 150/300/450 AP, there combo is going to deal double damage early/mid/late, and OHKO a squishy champion that didn't purchase defense, a squishy champion relying on things like range, speed, or something like Wind Wall to survive long enough to do their job.
As a support play this is pretty nifty to know, I've been able to turn around and KO a lot of cocky squishy champions at half health with the likes of full-tank Leona.
Everyone that isn't a decidedly late-game champion can usually kill any squishy in 2 full rotations. Tanks in 2 full rotations if you double that burst with tons of damage.
TLDR: read up guides on how to play Yasou, what his power spikes are, and what his weaknesses are.
