With the success of the Z series from the Sony Xperia line and with the flagship Xperia Z3, it is really only a matter of time before the Xperia Z4 was revealed. It doesn't look like this is the case, but there will be a Z4 unveiled in Japan. As far as the specs are concerned, the Z4, set for a June release date in Japan, is identical to the newly announced Z3+, which also has a June release planned for the United States.
According to The Verge, Japan already has announced their newest of the Z series smartphone last month with what they call the Z4. Stateside, it will be the Xperia Z3+, and it is essentially a slight redesign of the Xperia Z3 with an external upgrade and several refurbished internal upgrades. Like the Z3, the Z3+ is waterproof, and it no longer requires its micro-USB port to have a flap to protect from water. The port has also moved from the top left to the bottom of the phone.
Most of the specs have remained the same. The Xperia Z3+ has a 5.2 inch screen that has a resolution of 1080p, which isn't really that high considering the Samsung Galaxy S6 and LG G4. The rear camera is a 20.7 Megapixel job with Exmor RS sensor, the front-facing camera is a definite upgrade from the 2 Megapixel one from the Xperia Z3, as the Xperia Z3+ is about 5 Megapixels. What is kind of odd is that the Xperia Z3 was 7.3 mm thick and the Xperia Z3+ is 6.9 mm thick. One would think that the plus-sized Xperia Z3 would be thicker.
As far as the interior improvements, the Xperia Z3+ has been upgraded to the newer Snapdragon 810 with Adreno 430 graphics with 3GB of RAM. That is a definite upgrade from the Xperia Z3's Snapdragon 801, but unfortunately the battery has been slightly downgraded to 2,930 mAh (the Xperia Z3 was 3,100 mAh). According to the official Phone Arena specs, the Xperia Z3+ will come with Android 5.0, but it really should be easily upgradable to 5.1.
Engadget calls the Xperia Z3+ "a familiar flagship", and it is odd that Sony didn't just put out a new Xperia smartphone and call it the Z4, but named one of their tablets that. Last year's Xperia Z3 did pretty good in its sales, but since Apple and Samsung really put their best foot forward to completely transform their phone, it is not known how well the Xperia Z3+ will do in today's market.
The Sony Xperia Z3+ is expected to be available globally in June, along with its sister Z4 tablet that was announced at the last Mobile World Congress. The Sony Xperia Z3+ will be available in an aluminum framed body with a choice of black, white, copper, as well as a choice of refreshed aqua green.