Sony Xperia has confirmed on the official twitter account that Sony Xperia M4 Aqua will not get the Android 5.1 update. A lot of users using Sony Xperia M4 Aqua were waiting for the Android 5.1 update.
Sony Xperia M4 Aqua runs on Android 5.0 version at the moment and it will skip Android 5.1 update. The phone will directly get the Android 6.0 Marshmallow.
While we wait for next Android Marshmallow update for M4 Aqua, Sony has already updated the Android Lollipop 5.0 firmware of the Xperia M4 Aqua (E2303) and M4 Aqua Dual (E2333) in the Nordic region. You can upgrade your smartphones to the new build versions 26.1.A.3.92 for E2303 and 26.1.B.3.91 for E2333. There is no information about the update for other Xperia M4 Aqua variants, so this might only be region specific. You download the new FTF firmware files from the below-mentioned links.