The Sony Xperia Z4 looks more like an incremental upgrade than a radical one compared to the Xperia Z3. The design seems to have remained mostly the same, while the base storage has doubled to 32 GB, and the device now has a 5.1MP camera in the front. The Xperia Z4 is slimmer than the Z3 (6.9 mm vs 7.3 mm), and the battery has also shrunk from 3100 mAh to 2930 mAh. It remains to be seen whether this will have an impact on battery life.
More information here at Tom's Hardware.