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(Pocket-lint) - It's official, Sony has quietly unveiled its Xperia Z4 smartphone in Japan.

The Sony Xperia Z4 was delayed for release at Mobile World Congress in Barcelona, says Sony in Japan. Hence the quiet release. So what's new?

The Sony Xperia Z4 will come with a slimmer body than the Z3 at 6.9mm rather than 7.3mm, making it 144g rather than the previous 152g. It's also ditched the USB port cover but remains completely water resistant. All that and the Z4 is surrounded by a metal frame too.

The Xperia Z4 will have a 5.2-inch 1080p screen for 424ppi, a 64-bit octa-core Qualcomm Snapdragon 810 processor and 3GB of RAM. Onboard will be 32GB of storage but microSD means a further 128GB can be added.

The Sony Xperia Z4 camera is 20.7-megapixel f/2.0 with Sony's Exmor RS and Bionz processor. In the front is a 5.1-megapixel selfie shooter with 25mm wide-angle lens with "similar" image stabilisation as the rear camera.

The Z4 will come with Android 5.0 Lollipop and is powered by a 2,930mAh battery. It's IPX8 certified for water resistance and IP6X for dust, meaning it can be submerged in water to nearly 5 feet for up to 30 minutes and still work.

The Sony Xperia Z4 will launch in Japan this summer in White, Black, Copper and Aqua Green. Sony says it's not ready to announce if the handset will be available elsewhere yet.

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READ: Sony Xperia Z3 review

Writing by Luke Edwards. Originally published on 20 April 2015.