What are the specs on the PS4 PS? USFF PCs have managed it with silence up to 240W I think, maybe more. Custom Server PSUs up to 500, see Digital Storm Bolt (my personal desktop).
The current PSUs in most Mini-ITX can power the current Intel Quad Cores plus any GPU not requiring external power, but most of them are not silent. The Dell OptiPlex are. Just a matter of how much profit they want to make off the hardware. I'll vote for internal any day.
They've said the PS+ sub for MP now is partly to help with the value in offering the PS4 at $399 so I'm guessing they're not netting not a lot. Potentially even a small loss with how bad the yen tracks the dollar.