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(Pocket-lint) - Sony quietly announced the Xperia Z4 in Japan on 20 April, without the fanfare you'd expect for a new flagship launch. Well, it seems it isn't Sony's new global top-tier device. 

The Sony Xperia Z4 in its current format is going to be for Japan only, with another device lining up for global release later in May. 

These details come from a Russian source (via PhoneArena). The details state (translated) that "Sony Xperia Z4 will be available only in Japan," before going on to say that there will be another device for the international market, launching "in late May."

There are no specifications for this alternative device, but the name proposed by the source is "Xperia Z3+."

We've heard a couple of names used when talking about Sony's next-gen device. It's been called the Z4, Z3S, Z3 Neo as well as Z3+. These all suggest that the next device isn't a huge step change from the existing Z3, but that's yet to be confirmed. Whatever the situation, there's some confusion with naming.

It now looks like we're expecting to see something else from Sony Mobile later in May, but how different it will be from the Z4 that's already been announced, we don't yet know.

The Xperia Z4 is an incremental change from the Z3. It slims the design slightly, sticks to the same size display and resolution, but switches up to a Qualcomm Snapdragon 810 chipset and tweaks the front camera. It also adds a waterproof Micro-USB slot.

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READ: Sony Xperia Z4 vs Xperia Z3: What's the difference?

Writing by Chris Hall.