Sony mistakenly revealed the Z4 Tablet in its official Xperia Lounge application, though it quickly removed any evidence of the slate's existence.
Fortunately, XperiaBlog saved a couple of screenshots, which show that the Z4 Tablet will be extremely thin, sporting a "2K" display (probably with 1440 x 2560, or 1600 x 2560 pixels). Sony is mentioning that the slate features "leading battery performance", and "the latest ultra fast processor" (we assume this is a 64-bit, octa-core Qualcomm Snapdragon 810).
Not long ago, we heard that the Z4 Tablet's display would be a 10.1-inch one. Other features should include 4 GB of RAM, 32 GB of expandable internal memory, and Android 5.0 Lollipop. Like Sony's previous slates (including the Xperia Z3 Tablet Compact and the Xperia Z2 Tablet), the Z4 Tablet will most likely be dust- and water-resistant.
It seems that Sony plans to announce the Z4 Tablet on March 3 (on the second day of MWC 2015), though we might see the device in the flesh a bit earlier. Of course, it's rather weird for Sony to unveil the Z4 Tablet before the Xperia Z4 smartphone, but it seems that this is how things will go anyway.