When speaking with OPM, Neil Brown – Sony's R&D senior team leader – revealed that PS4 owners can play their digital libraries on any console.
If you visit a friend's house and wanted to instantly play a digital title in your own library, the instant play and background downloading feature will make it possible. To clarify the process Neil Brown described a scenario.
“You can visit your friend's 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."
Brown then added the instant play and background downloading, "makes a digital library a practical option in the real world."
Brown also added that through a system to Blu-ray discs, you will not need to wait for installations to fully finish.
“A similar system also works on Blu-ray, chunks are automatically copied to the hard drive in the background. 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.”
You won't play through an entire game if your PS4 continues to download, but it will at least remove the waiting process when first purchasing a digital title or when re-downloading digital games on a friend's PS4.