I need to make the phone/tablet show the shutdown/reboot dialog.... like pressing power-off - but it will be decision of user what to do... is that possible ?
You should be able to do that using Accessibility Services if I recall correctly.
You need to invoke the GLOBAL_ACTION_POWER_DIALOG using a performGlobalAction.
Here is a CodeLab from Google which you would need to adapt to B4A but it should be possible with the B4A wrapper I cited above.