The trend lines showed a year ago that a high end smartphone in about 3 years from now (mid-way through the PS4's lifetime) will have higher paper specs than a PS4 (paper specs, not necessarily as well-utilized).
An Nvidia Shield tablet is on paper roughly equivalent to an XBox360. The Shield set top box is double that. A few more doublings and the prediction will play out.
You may need a smartphone, though that's debatable, but you don't need a high-end expensive one. And those wont't have paper specs anywhere near PS4-level anytime soon.