Using ext4 formatted SD cards in CM11 is relatively easy, but there’s a lot of information on forums that seems to be wrong, misleading, or incomplete. This method worked for me:
Plug your SD card into a Linux PC.
Format the card to EXT4; either use gparted, or run something like
sudo cfdisk /dev/mmcblk0
# create a single primary partition
sudo mkfs.ext4 /dev/mmcblk0p1
Mount the card, and make the root world-writable:
sudo mount /dev/mmcblk0p1 /mnt/disk
sudo chmod 777 /mnt/disk
sudo umount /dev/mmcblk0p1
sync
Put the card back in your phone; if all is well, the folder
/storage/sdcard1/Android should have been created.
Removing and formatting the card with the phone turned off worked for me, though just unmounting/mounting may work too.