Can I use ext4 on my SD card in a non-rooted phone?
ext4 should be supported if you have an upgrade to 2.3 or 3.0 Android (I see that your post is from just before the 2.3 &amp; 3.0 releases...).

...and when btrFS is added to Android, it has a nice mount-option ",ssd" with wear-levelling etc that ext4 doesn't have... and more speed than ext4 now, esp if you use ",compress=lzo" OR ",space_cache" in fstab (but don't use compression WITH the space_cache option, only use 1 or the other or else it slows to a crawl in some benchmarks) -- and that speed comes despite btrFS checksumming of all data (not only of the journals, as ext4 does). :-)