Do ext4 filesystems need to be defragmented?

Do ext4 filesystems need to be defragmented?

Yes (but very rarely).

If so, how do I defragment them?

Copy all the files off the partition, erase the files from the partition, then copy the files back onto the partition. The file system will intelligently allocate the files as you copy them back onto the disk.

If not, could you post a simple explanation of why they do not need to be defragmented?

ext4 acts in a more intelligent way than merely adding new files into the next available space. Instead of placing multiple files near each other on the hard disk, Linux file systems scatter different files all over the disk, leaving a large amount of free space between them. When a file is edited and needs to grow, there’s usually plenty of free space for the file to grow into. If fragmentation does occur, the file system will attempt to move the files around to reduce fragmentation in normal use, without the need for a defragmentation utility.

Thanks to a Comment by @Green Reaper my attention has been drawn to e4defrag.

I have been using ext4 partition for over a year. I often ran out of space, moved a lots of files in and out etc etc... these things are bad for fragmentation and yesterday I checked for fragmentation for the first time and only 2 files (not %, just two files) were fragmented.

On the Windows partition that was used for about a year longer with much more free space available, I have 95% fragmentation.

So no, you really don't need to defragment ext4 and if you want to be sure, leave the default free space for ext4 (default is 5%, can be changed by ex2tunefs -m X).