Traditional, Unix-derived, file systems, like Ext3, use a indirect block mapping scheme to keep track of each block used for the blocks corresponding to the data of a file.
This means that you can improve the performance, storage limits and features of your current filesystems without reformatting and/or reinstalling your OS and software environment.
If you need the advantages of Ext4 on a production system, you can upgrade the filesystem.
The procedure is safe and doesn't risk your data (obviously, backup of critical data is recommended, even if you aren't updating your filesystem :).
The performance is much better, and the fragmentation is much improved in some workloads.
Fsck is a very slow operation, especially the first step: checking all the inodes in the file system.
Ext4 will use the new data structures only on new data, the old structures will remain untouched and it will be possible to read/modify them when needed.