lets say i have a char in inferno, someone creates a game where he kills all 4 bosses (just them, no other quests), will be this way possible for me to do all the other quests in inferno later, if i so desire?

You still have to.do them in order to open them up

i mean smth like this http://eu.battle.net/d3/en/forum/topic/4551989139
will be whole inferno difficulty complete?

If you have some one do Butcher for you (you have not done anything in inferno yet), you'll be able to make a game at the butcher quest which will allow you to access any way point before the butcher. But if you wanted to kill King Leoric all of a sudden, He'd already be dead if you started at The Butcher and you can't change quest to Skeleton King cause you have not don't that quest yet or progressed there from the beginning.

Example, when you click change quest it would show up something like this

Act 1
1.) The Fallen Star
9.) The Imprisoned Angel
10.) Return to New Tristram

Act 2
1.) Shadows in the Desert

So with that example, if you wanted to go back and do the Skeleton King quest which is 4.) Reign of the Black King
You'd have to start at 1.) The Fallen Star and progress your way there or have a buddy who is at/has done that quest make the game and you join him.

Theoretically you could have chars set up to do the final quest of each point and just get them to "resume" their game and join, they go away, you finish last little bit and bamm that section is done.

Not sure I would go to that much effort though, talking what 12 chars per act?