1: What did you use to create the Ext4? Not CWM I hope?
2: Go to Settings->Performance->Mounts2SD and make sure your file system type is set to Ext4 and not Ext3 or Auto.
3: Try disabling the Journal. If you did not manually do this when you created the file system, it will reduce a lot of writing to the partition.
Wow! These noticeably sped my phone up! Great tips!