Recently Xperia C4, Xperia C5 Ultra and Xperia M5 started receiving Android 5.1 Lollipop update unexpectedly. So the Xperia M4 Aqua users waited till they get the Android 5.1 update. But unfortunately Sony Mobile has stated that Xperia M4 Aqua will not receive Android 5.1, instead go directly Android 6.0 Marshmallow on the twitter page.
So now Sony Mobile has rolled out a minor updated with the build numbers to 26.1.A.3.92 for E2303 and 26.1.B.3.91 for E2333. The update is live on Nordic & Baltic regions for now. It will go worldwide shortly.
@KarCerKK Regarding your question the Xperia M4 Aqua will skip Android 5.1 and will receive Android 6.0 (Marshmallow).— Sony Xperia (@sonyxperia) February 2, 2016