I suspect it will output 3840 x 21260 and not 4096 x 2160 but I do not know for sure. All we have are press releases from Sony, no spec sheet that I can find, and press reports. Some press reports say 3840 x 2160 and others say 4K. But I suspect most of these gadget guys filing reports don't know precisely what resolution 4K is exactly anyway. I really think consumer 4K will be 4 times the number of pixels in a 1920 x 1080 HD frame. Of course I don't know but the 4K flat screen panels being annouced are 3840 x 2160 and I think any upscaling Bluray player will be designed to do that.
How consumer 4K discs will be, I do not know but even here I would expect 3840 x 2160 too. I think true 4K will be reserved for the commercial movie theater industry, but I don't know.