Sony just announced its new flagship mobile image sensor, the Exmor RS IMX230, which is expected to arrive in April, 2015. It is likely that the Xperia Z4 will use this sensor and could be released around the same time.
The IMX230 sensor has a 21 megapixels resolution and comes with a number of new features. Sony claims that it is the industry's first CMOS image sensor built for smartphones that can track fast-moving subjects. This is in part down to its built-in image plane phase detection autofocus. The sensor will also support High Dynamic Range for high resolution still images and 4K video.
Sony has already scheduled when its press conference will take place at the upcoming International Consumer Electronics Show (CES) 2015 in Las Vegas. The company will take to the stage at 17:00 PST on January 5th, 2015. The Xperia Z4 is likely to be announced then.
It doesn’t look like Sony is to abandon its rapid release schedule. The Xperia Z3 has only been out for a few months at this stage, but it was released just six months after the Xperia Z2.