Sony released the Xperia M4 Aqua in June 2015 and despite still being a mid-range smartphone, it was a significant improvement over its predecessor as it integrated some of the features from Sony's flagship smartphones such as capable cameras, a long-lasting battery and waterproof hardware. Its design also resembles that of the flagship devices at that time, although it is built with less expensive materials to keep a lower price. Also, it was one of the first devices that even as it is IP68 for water and dust resistance, it features a capless micro USB to charge the phone or connect it to other devices. The phone was running Android 5.0 out of the box and it is listed as one of the devices that are going to be updated to the most recent version of the Android operating system called Android 6.0 Marshmallow.
Sony released a Marshmallow Concept software for some of their devices, the purpose of such a move was so users could test the software and its features and report back at them if they found any bugs or issues to be solved before the final release. Those selected devices are still running that conceptual build, but it was said to work until early 2016. This made us believe that the Marshmallow update for some Sony devices could be rolling out soon, but so far, there's not a single device from the company that has received such update.
Many Xperia devices that were released last year were updated to Android 5.1 Lollipop, which brought some minor changes to the UI. Some others were unexpectedly updated to that version recently, but apparently, that won't be the case for the Xperia M4 Aqua. The company mentioned on its official Twitter account that this smartphone will be updated to Android 6.0 but it will skip Android 5.1. The company obviously didn't mention a specific timeframe for the rollout of such update, but it has been reported that this handset received a minor update in the Nordic region. It might have been a region specific update as other variants of the Xperia M4 Aqua haven't received any firmware updates.