What exactly is the problem? I think the card mounts but with the wrong permissions, so Code: ls -l /media/sdcard doesn't say this? Code: drwxr-xr-x 2 nemo nemo ... If so you might be able to change it, but it probably won't stick when you reboot... Code: chown nemo:nemo /media/sdcard/"label"/ What device are you using and why did you decide on ext4?
ls -l /media/sdcard
drwxr-xr-x 2 nemo nemo ...
chown nemo:nemo /media/sdcard/"label"/
total 8 drwxr-xr-x 2 nemo nemo 4096 2017-10-20 15:32 2205-6F10 drwxr-xr-x 3 root root 4096 2017-10-21 19:24 "label"
chown: changing ownership of '/media/sdcard/"label"/: Operation not permitted