I performed the tests on /scratch partition writing the log in /. I did it twice one mounting and unmounting the fs at each test so to delete any trace of information from the buffer cache and one leaving the fs mounted between tests. Tests were automatically repeated for sizes from 64kB to 4GB and record length between 4kB - 16384kB. Iozone automatically doubles the previous sizes at each test (4GB is the smallest multiples smaller than the 5GB file size limit I set).
From the numbers ext4 performs much better in writing while reading is basically the same if not slightly worst for smaller files. There is a big drop in performance for both file systems for the 4GB size.
What however I find confusing is that I did the tests again setting the max size of the file at 100M and doing only write tests and ext3 takes less time despite (22 secs vs 44s in this case) despite the numbers saying that writing is almost 40% faster there is something that slows the tests down (deleting?). Speed of tests become similar for sizes >500MB they both decrease steadily until they finally drop at 4GB for any record length in both file systems.
Below some results mostly with the buffer cache because not having it affects mostly ext3 for small sizes of file and rec length as shown in the first graph.
EXT3: write (NO buffer cache)
EXT3: write (buffer cache)
EXT4: write (buffer cache)
EXT3: read (buffer cache)
EXT4: read (buffer cache)