Though Sony has already released the PS Vita, both the handheld console and the upcoming PS4 were developed simultaneously according to Sony's president of Worldwide Studios, Shuhei Yoshida.
In the latest issue of Edge, Yoshida explained that work on the PS4 started as soon as the PS3 was released.
"As soon as we launch a platform, we have small number of tech people move on to planning for the next. As soon as the PS3 launched in 2006, I'm sure some parts of the company, especially those hardware people who work in R&D and the semiconductor [area], must have started the R&D effort."
It wasn't until 2008 that full development of the PS4 really began, however, and it was then that a "larger, more cross-sectional team" began working on both the PS4 and the PS Vita.
"PS Vita was a shorter project compared to PS4," said Yoshida. "but Mark and Worldwide Studios were involved in both projects, and we worked on both projects simultaneously."
So it seems that cross-platform control that Sony was so keen to advertise during the PS4 announcement is for a reason - these two platforms were developed together.
Interesting, probably means PS Vita + PS4 will work well together (crossplay, remote play, second screen). Hopefully Sony can manage to create something significant for both platform when used together, unlike what they have done with PS3/PSP/PS Vita integration.