While researching EXT4 performance, I came across this website:
They were quite informative. If you are interested in learning how EXT4 compares to EXT3 and other major filesystem competitors of the day (XFS and ReiserFS), have a read. One caution, read their bar graph summaries/legends carefully because the graphs can be misread if you don't pay careful attention.
EXT4 does not win at everything but it seems to hold the best place in more tests than its competitors. Well, at least in these tests.
For those of you interested in next generation filesystem coming our way, Btrfs, they also have benchmarks for the current development snapshot.