Here is another good tutorial on ext4:
It explains that you can mount your existing ext3 partition as ext4 for slightly improved performance. It also describes how to convert an ext3 partition to ext4.
I have read that if you are concerned about possible data corruption in the event of a system crash due to the delayed allocation feature of ext4, you can add nodelalloc
to the partition's boot options in fstab. Although I have read that this is not really necessary with the most recent linux kernels that have better implementations of ext4.