How mount (or auto mount) an usb stick formated with ext4 and be visible or working with apps on Android 5 , Marshmallow and later versions
We still need root. First I need make a dir in /data/media/0/ (/sdcard) 

mkdir /data/media/0/usb1/


which means usb is not detect as usb ever but I can read data from apps that it is what I want .

After I made a script with :

su -mm -c "mount -t ext4 /dev/block/sda1 /data/media/0/usb1/"; 
su -mm -c "mount -t ext4 /dev/block/sda1 /storage/emulated/0/usb1/"


and after with https://play.google.com/store/apps/details?id=os.tools.scriptmanager

I made a favorite script and with the add-on to create a launchers, I made a icon in home screen (shortcut I preferred instead widget) and on click it, it mount my ext4 USB pens.