No. Steam games run on PC/mac and not on Sony PS4 (or Nintendo or xbox) at all.
Steam games can be shared between computers. Up to 10 friends or family members can do this by logging into Steam on the computer to enable sharing.
no steam retail codes cant be reused
At this point neither Valve nor Sony have released any details for the PS4 to support the Steam Machine controller.
Nope. But you can transfer it to pc
You cannot play PS4 games on a PS2, as they are incompatible due to different hardware and software architectures. The PS2 is designed for games specifically made for its system, while PS4 games require the more advanced technology of the PS4 console. To play PS4 games, you need a PS4 or a compatible device that supports them, such as a PS5.
once
No you will not be able to play the games either way. Systems changes on the PS4 allow them more design capability for games and many titles will be released in a downgraded PS3 version. You still will have to have a PS4 to play PS4 games and a PS3 to play PS3 games.
Through steam pipes.
no you cant steam friends in an acount.
No, the PS4 does not have backward capability and cannot activate (play) PS3 or PS2 games.
Yes you can, you import and add non-steam games to your Steam library. From within the library you can launch and run any games, although 'Steam achievements' will only be available for Steam enabled games.
They don't. Only games distributed and managed by Steam can make use of the Steam API.