Sony announced a list of devices, that will receive the update to Android 5.1 Lollipop, last year in June. Back then, there was a wave of disappointment that the Xperia M4 Aqua won't get the software.
It turns out the phone OS will jump directly to Marshmallow 6.0. Sony has confirmed this statement.
M4 Aqua will skip Android 5.1.1 update going straight to the newest installment.