Android - 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 & 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). :-)
I actually had my sdcard on ext4 and got it mounted permanent at boot time (I have root on my phone).
The biggest problem was that the apps don’t expect permission checking on /sdcard, but extX enforces this (no mount option around this). So if one app stores e.g. some audio there and calls another subcomponent to play it, it fails.
The same is true for photos (camera shots them, some previewer might want to access them).
You can manually fix this, but you have to run after every new file created... There would be an nice real solution to this, but it requires system modification: let the apps run with umask(002) instead of 022. Android is UPG (User Private Groups), so there is no big harm in doing so. – Now the apps could choose if they want to expose files to others by setting the group to “sdcard“, or not. This would also add to an enforced separation where the apps want it.
It appears that the answer is "no" :(. I was asking because I have a new card to go in the phone, so I thought I'd just try it -- it didn't work. When I ask the phone to mount its card, it goes straight to saying "SD card safe to remove."