While I do find cross game chat a huge factor in 360's great success this gen, I do not think that inclusion alone will bolster PS4. Diminishing sales by not having it, yes. Now if PS4 has the same featured offerings as Xbox 3 it will sell a lot more unless Microsoft creates more games that become hits because right now Sony has the large stable of studios and exclusives. Chances are though both systems will have their own unique offerings, but basic things like cross game chat will be standard as well as the basic use of the console.
Before the PS3 everyone was nice to me :(