Latest Update: The Xperia Z4 has now been officially announced for the rest of the world, under the name Sony Xperia Z3+.
It might not seem that long since the Xperia Z3 was released, but with Sony's rapid release schedule for its flagship smartphones we now have Sony Xperia Z4 and Sony Xperia Z3+.
For those in Japan the handset is known as the Xperia Z4, because Sony's huge following means it can get away with a new name for such an incremental upgrade.
It's not trying to pull the wool over the eyes of the rest of us however, with the Japanese firm opting for the more subtle "Sony Xperia Z3+" moniker for the handset's global launch.
While Sony's six-monthly reboots seem rather excessive, it does mean it has a fresh new handset in the Xperia Z4 (and Xperia Z3+) to give the Samsung Galaxy S6, HTC One M9 and LG G4 a run for their money.
Cut to the chase
What is it? Sony's next flagship smartphone
When is it out? June 2015
What will it cost? A lot - this will be a high-end device
Sony Xperia Z4 release date
The Xperia Z4 will be launching in Japan this summer, while international availability of the Xperia Z3+ is pegged for June 2015.
Sony Xperia Z4 design
Now that we've seen the final products, we know the devices aren't too big a departure from previous iterations of the Xperia Z line up - with Sony sticking with its distinctive Omnibalance style.
At just 6.9mm thick, with a 144g glass-and-aluminium body, the Z4 and Z3+ feature the same water-and dust proof features the Xperia Z line up has become famous for in a slimmer body than the Xperia Z3.
The really good news here though is the fact the Xperia Z3+ and Xperia Z4 don't have a cover over the USB port - making them at lot easier to charge.
Sony Xperia Z4 screen
Both the Xperia Z4 and Xperia Z3+ have a 5.2-inch 1920 x 1080 display, despite early rumors hinting the screen would be 5.5 inches.
Sony Xperia Z4 camera
Sony hasn't changed anything in terms of the rear facing camera on the Xperia Z4 and Xperia Z3+.
Both handsets continue to use the firm's 20.7MP lens which has been present in Sony's flagship phones as far back as the Xperia Z1.
The front facing camera is actually slightly better, with a 5.1MP resolution and a 25mm focal length for better wide-angle selfies.
Sony Xperia Z4 OS and power
One of the main upgrades for the Xperia Z4 and Xperia Z3+ comes under the hood, with the new flagships benefitting from an octa-core Snapdragon 810 processor.
This is backed up by 3GB of RAM, providing a top notch level of power, which should mean Android 5.0 Lollipop runs nice and smoothly.
Sony Xperia Z4 battery
Slightly disappointingly the Xperia Z4 and Xperia Z3+ pack smaller, 2930mAh and 2900mAh power packs respectively, while the Z3 boasts a 3100mAh battery.
Technically the Snapdragon 810 is a more power efficient chip, and with screen resolution and type staying the same we're hoping the drop in battery size doesn't have too much of a negative effect.
Sony Xperia Z4 rivals
Sony's biggest challenges come from the Samsung Galaxy S6, HTC One M9 and LG G4. Three big name, high spec devices which have a head start on the Xperia Z4 and Z3+.
Plus the OnePlus 2 - which is due out later this year - could be a tempting alternative, as it's likely to offer similar specs at a lower price.
Sony Xperia Z4 cost
There's been no information on how much the Xperia Z4 or Xperia Z3+ will cost, however as flagship smartphones (and Sony ones at that) we'd expect them both to ship with a high price tag.
The Xperia Z3 launched at £549 ($630, AU$849) and we'd expect the Z4 and Xperia Z3+ to launch at around the same price.