I actually am not sure if it defaults to ext4. I found this thread when I was about to post the same thing asking in the Dinc forums. I run CM7 RC4 and a simple mount command from the terminal shows ext3 instead of ext4.
For me /system and /data are yaffs2 and /data/data is ext3.
EDIT: Looked at it again, /system and /data/data are yaffs2 and /data and /cache are ext3. Anyone know how to format in CWM?