A firmware update recently started to roll for the Xperia M4 Aqua that moved the build number from 26.3.X.0.131 to 26.3.X.1.33. The main noticeable addition is that the Android security patches have been updated from April to July 2016. The update is live across all six variants of the Xperia M4 Aqua and you can find FTF firmware files below.
DOWNLOAD Xperia M4 Aqua FTF Firmware Files – Android 6.0.1
Xperia M4 Aqua (E2303) 26.3.A.1.33 Switzerland Generic
Xperia M4 Aqua (E2306) 26.3.A.1.33 Latin America Generic
Xperia M4 Aqua (E2353) 26.3.A.1.33 Thailand Generic
Xperia M4 Aqua Dual (E2312) 26.3.B.1.33 Singapore Generic
Xperia M4 Aqua Dual (E2333) 26.3.B.1.33 Russia Generic
Xperia M4 Aqua Dual (E2363) 26.3.B.1.33 India Generic
Thanks Adian, César, Maxine, Mark, Mice and Niclas!