Ext4 partition is used together with data2ext scripts in phones with low internal memory. This effectively uses the space in SD card as a data partition, where applications are installed. It requires though a very fast SD card (minimum a class 4 card).
I've seen more commonly used ext3 partitions, which are used with app2SD+ scripts. These don't require fast cards and are still able to move more applications to SD card than the standard Google App2SD.
I found Gingerbread ASOP ROMs working very nicely with app2SD+ scripts.