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I'm sorry to hear that but
http://www.sonymobile.com/global-en/products/phones/xperia-m4-aqua-dual/
Waterproof*. Dust tight. Worry-free.
The Xperia M4 Aqua Dual is as durable as it is beautiful. Its innovative design protects it against water as well as fine dust particles so you’ll never have to put it away ‘just in case’.
With that said, contact your Local support and request a vacuum test that will prove who is at fault, either you have a defective unit OR you didn't firmly closed the covers and/or there are cracks