Major practical differences between ext3 and ext4 - if any?

Assuming there's no need to worry about compatibility with older systems, I expect it's always better to use the latest stabile linux file system, which currently seems to be ext4.

However it seems ext4 isn't much of a change. Wikipedia ext4 article does list 11 main features for ext4, but as far as I understand most of these don't appear to have any major practical difference to ext3. The article also discusses a potential data loss caused by delayed allocation, but to my understanding this would happen only in esoteric system crash cases and thus not make any practical difference either.

Is this just a small gradual upgrade without any major differences, or should I be running to convert my ext3 systems into ext4?

EXT4 above all says it will boot your OS faster. Ubuntu 9.04 should be booting up to 30% faster. If this is the only practicall use I would be stunned, since my Ubuntu boxes boot so fast I cannot even go get me a cup of coffee...

But you can use larger harddrives, have larger files, more security is built in to safeguard data from crash and last but not least, it is being optimized for speed...

When starting to develop EXT4 in 2006 it was meant as an improved EXT3. However, changes where to big to just call it an upgrade. Thus the new name. It was added to the kernel in 2008...

I'd advice you to wait a while before you switch to ext4, especially if you're on Ubuntu. There are nice performance and fragmentation related improvements in ext4, but it's not worth any system instability.

Read the blog post by Ted Ts'o, the ext4 maintainer, on the potential data loss due to delayed allocation: Delayed allocation and the zero-length file problem. Also have a look at the three ext4 related issues present in Ubuntu 9.04 (Jaunty) and laid out in the 9.04 release notes.

Ext4 is the default filesystem on Fedora 11, and they have a nice FAQ on Ext4 in Fedora 11 that answers your question quite well. If on Ubuntu, I'd wait till Karmic (which will also have Ext4 as the default fs) before switching.

Short answer: no rush.

Longer answer:

I don't think it's always as simple as "use the latest release of a file system." Alos, as pointed out by @nagul, I don't think ext4 is more stable than ext3 at this point.

Even speed difference comparisons are not straightforward. Different file system types tend to perform better in different situations. There's a good linux-mag article that compares several types of file system performance.

One of the major questions you have to ask yourself is does this fs need journaling? I typically setup my machines to use ext2 for /boot and other other partition I mount read only, and /tmp (for speed).