It looks like the Sony Xperia M4 Aqua, along with the M5 are already getting the Marshmallow 6.0 update. The update is available internationally.
The build number of this software is 26.3.A.0.131 for the M4 and 30.2.A.0.100 for the M5.
THe availability for M4 is rather limited right now, so you need to be patient.