The question appears in variate forms  just to say it is ridiculous on Android that ext4 is the default file system doesn't recognize usb pen formatted with ext4 (but android 4.4 reconize it without any problem), as a Linux user this is a big regression , and I still not found a way to usb be detect as usb , like pens with FAT32 or NTFS. I have root on my device .
We still need root. First I need make a dir in /data/media/0/ (/sdcard)
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/"
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.