One of the biggest complaints about the PS3 was the lack of cross-game chat. Even though Sony teased such functionality in a pre-launch E3 demonstration of the PS3 UI, it took years for Sony to admit such a thing wasn't technically feasible on the system. Cross-game will indeed be possible on the PS4, however, for up to eight players at once.
The total amount has been confirmed by Sid Shuman on the PlayStation Blog in a Q&A about Killzone: Shadow Fall. Shuman said that the PS4 will support eight players at once in a cross-game party chat, just like the PlayStation Vita.
This is an important feature for many, and one that gamers have long been used to on the Xbox 360.