The PlayStation 4 has appeared on a Sony engineer's LinkedIn profile.
Frederick Umminger, who is a software engineering manager at Sony Computer Entertainment America, clearly displays the PlayStation 4 on his CV. He has been working on maintaining "voice chat library for PlayStation 4″ ever since June 2010.
Should anyone be surprised by this? The short answer is no. Sony, as is the case with Microsoft and Nintendo (Wii U), has naturally been pumping hundreds of millions into the development of their next-generation console, and that's not even considering the R&D (Research and development) costs -- the company has spent several billions in the R&D department alone for the PlayStation 3. Either way, the unannounced platform will definitely be missing an E3 appearance this year with Sony going on record to state they won't be revealing hardware news at the expo.
What may come as a surprise, however, is the nature of how the PS4 in this instance has been outed. This is arguably the next best thing to Sony actually officially announcing the PS4.