was installed with 16GB default, on my vsphere i added 4GB.
Where is Physical volume of 4gb? your pvdisplay command shows only 2 PVs - 16gb and 1mb. create physical volume of 4gb first.
# pvcreate /dev/sdxx
Then, add newly created PV to volume group.
# vgextend media /dev/sdxx
Verify it - you should be shown 4gb as VFree space.
Since you are extending root LV, boot in rescue mode and follow the steps
# lvm lvextend -l +100%FREE /dev/media/root
# resize2fs -f /dev/media/root