EXT4 is usually pretty good at keeping files fragmentation at minimum, but, sometimes, especially if dealing with really huge files, some fragmentation may actually occur.
Luckily EXT4 supports online defragmentation, command fsck displays, among other things, fragmentation percentage:

[root@fedora ~]$ fsck.ext4 -fvn /dev/sda1
e2fsck 1.42.12 (29-Aug-2014)
Warning!  /dev/sda1 is mounted.
Warning: skipping journal recovery because doing a read-only filesystem check.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information

         429 inodes used (1.31%, out of 32768)
           **5 non-contiguous files (1.2%)**
           1 non-contiguous directory (0.2%)
             # of inodes with ind/dind/tind blocks: 0/0/0
             Extent depth histogram: 420
       45161 blocks used (34.46%, out of 131072)
           0 bad blocks
           1 large file

         402 regular files
          17 directories
           0 character device files
           0 block device files
           0 fifos
           0 links
           1 symbolic link (1 fast symbolic link)
           0 sockets
         420 files

The command e4defrag, which is contained in e2fsprogs, can be used to perform online defragmentation of EXT4 volumes.

[root@fedora ~]$ yum install e2fsprogs

Then sit tight, prepare to wait some time and run what follows to defrag the volume:

[root@fedora ~]$ e4defrag -c /dev/sda1