Create a partition on SD card for ext4 filesystem
OS X cannot see ext partitions. You would be better to do this on Linux.

You can install support e.g OS X Fuse as outlined in http://osxdaily.com/2014/03/20/mount-ext-linux-file-system-mac/

You will also need FUSE-Ext2 as https://github.com/osxfuse/osxfuse/wiki/Ext

I am not sure if this will allow you to create ext4 partitions, but you can certainly mount them.