Microsoft executive Albert Panello confirmed that PlayStation 4 can be connected to Xbox One through the latter's HDMI-in port, thus allowing PS4 games to run within an Xbox One overlay.
Panello also confirmed that, using this setup, players can multi-task between the two rival consoles. "Any application can be snapped to a game... so if you wanted to be playing Ryse and Killzone at the same time, you could snap that," he said.
(Killzone is a PlayStation exclusive while Ryse is an Xbox exclusive.)
Evidently, any console that outputs to HDMI can be connected to Xbox One in the same mannar. Such consoles include Xbox 360, PlayStation 3 and Wii U.