Installed redmine image on 5GB host but df shows only 3.8 GB in system total

I've installed redmine from iso on KVM host, all installation went fine but now df -h shows

root@redmine ~# df -h
Filesystem                Size  Used Avail Use% Mounted on
rootfs                    3.8G  1.2G  2.4G  34% /
udev                       10M     0   10M   0% /dev
tmpfs                      50M  172K   50M   1% /run
/dev/mapper/turnkey-root  3.8G  1.2G  2.4G  34% /
tmpfs                     5.0M     0  5.0M   0% /run/lock
tmpfs                     100M     0  100M   0% /run/shm
/dev/vda1                 228M   17M  199M   8% /boot
 

Where did the additional space go? Did I do something wrong during the installation making the rest of the space unavailable?