It looks like the PTCRB has just certified Android Marshmallow for the Xperia M4 Aqua and M4 Aqua Dual. A new update with build number 26.3.A.0.131 has appeared on the PTCRB page, which compares to current build 26.1.A.3.111 (Android 5.0 Lollipop).
The move from ‘26.1’ to ‘26.3’ based firmware is very likely to indicate that this is the long-awaited Marshmallow build. We hope that the update includes adoptable storage, as it is a phone that really needs it, especially the 8GB variants. We’ll let you know once it starts to roll, it shouldn’t be too much longer now.
Thanks Julius, Marko and xezrunner!