Xperia M4 Aqua reboot-bootloader only shuts it down I'm trying to reboot my Xperia M4 Aqua into bootloader mode but all it does it turn it off and I get the battery charging image as if the phone is shut down and in charge.

I tried using the commands

adb reboot-bootloader 


and

fastboot reboot-bootloader


and I also tried doing it manually by pressing volume up button while inserting the usb cable in... same results.

The command

adb devices


returns the device while

fastboot devices 


doesn't

OS is Windows 10
Thanks for enlighting me; I always use without hyphen

adb reboot bootloader

which is in line with syntax, but for curiosity tried couple of moments ago on Moto G4 with hyphen successfully

adb reboot-bootloader

But, in what mode you issued command 

fastboot reboot-bootloader

Because you are using Windows platform, acquire appropriate drivers for that device.

In developer options, both Enable OEM unlock &amp;&amp; USB debugging must be enabled.

Regarding your problem, I briefly looked for same or similar, excluding Samsung and found

LGV20, Wayteq xTab100qcr and Glass.

I'm quite interested in this issue, though never worked with Sony, so specifics of their devices are unknown to me.

Try with volume- or volume+ button combinations for entering fastboot mode.

If that doesn't help, look at this.

I remembered I actually had contact with Sony device - it had some sort of custom FRP - my Xperia Theft Protection (MXTP) - but haven't worked with it. I planned to use Xperia FlashTool.

From Xperia FlashTool you should be able to reboot to bootloader.

Please, do inform what you have done.