According to Sony's R&D senior team leader, Neil Brown, the PS4 will allow you to head to a friends house, log into your account, and play any digital game you own.

Heres the full quote from brown:

“You can visit your friends house you can log into your account and play any game from your digital library, which is good. But how useful is that if it takes half a day to download the game you want to play? With Play As You Download you get much quicker access to at least the first section of the game so you can start playing quicker. So this makes a digital library a practical option in the real world.”

“A similar system also works on Blu-ray, chunks are automatically copied to the hard drive in the background. This means that after the first few minutes your game can rely on having faster read speeds from the hard drive. Which provides a better experience for players, and this is a completely background process for the player. They don’t have to wait for anything to install before playing the game. The game will launch as soon as the disk has been put in the drive.”

Source