So, the rumors were accurate - Sony has actually developed the world's first 4K2K (3840x2160) OLED TV panel, and unveiled a prototype unit. The panel is 56" in size and uses an Oxide-TFT and Sony's own "Super Top Emission" OLED technology.
Sony wouldn't reveal any technical specification - they only said that this panel "delivers all the features expected from OLED TVs, such as high contrast, brightness, rapid video image response time and rich images produced even at wide viewing angles". But that's just as well - Sony wouldn't say anything about a release date (or expected price) of this display either. They just said that "Sony will continue to research OLED technology with the goal of commercializing its new 4K OLED televisions".
Sony did say that the panel was jointly developed with AU Optronic (AUO). This collaboration was actually rumored back in March 2012. Back then it was reported that AUO will provide the capital and manufacturing while Sony contributes its OLED technology. Sony is also collaborating with Panasonic on OLED TV production technology, and in September 2012 they promised us OLED TVs by 2014. We're guessing that Sony doesn't have any plans to release this 4K OLED any time soon...
Perhaps Sony's collaboration with Panasonic indeed focuses on new production technology, and in the meantime the company is also developing larger panels based on their current process, together with AUO. Hopefully we'll learn more soon.
Source: Sony