So what are the keys to Kyrat for then?

this is a really cool feature to be added to the PS3.5

That actually adds co-op itself to the game
EDIT: What Bananaking says

keys to kyrat dont do streaming. you actually have to download an "App" which is the farcry4 game and you can access it with a key/code. no streaming happens, your PS4 is running and rendering everything.

with this your friends PS4 is running the game and sending a stream to you. 

Neat feature. Helpful in trying new games without downloading the huge demos.

Just watched the other video Ethomaz posted and I definitely like how Share Play is looking.

Yeah I just realized that as well. This is truly in every sense of the phrase, literal and otherwise,  a game changer. This feature I fell truly, as they say, is for the player. They have to be losing out on game sales because of this. For that fact I applaud Sony for offering this, what appears to be, great service.  This justifies the cost of PS+ alone.

For every game shared there will be a game bought on the back of being able to demo EVERY game. In fact I'd say this could actually help sell games.

Yeah, this is an awesome way to sell games. If someone in my friends list is on the fence about a game like CoD, I can just let them play the whole game as a demo. This could truely work out great for all parties involved.

I agree. It's a great way for PS4 users to try games out before deciding to purchase their own copies.