Error = setup (hd0,4)
Above also misses a part that could help:
grep -v rootfs /proc/mounts > /etc/mtab
Can also help if in livedvd to:
mount /dev/sda3 /mnt/gentoo
mount /dev/sda1 /mnt/gentoo/boot
Note sda3 in example = root partition so change to your correct numbering / label and then do the grep and then grub like above to get correct method / partitions but depends on linux distro
Run livedvd and run installer again then select upgrade and grub / boot menu option only then select the only listed partition in the drop down that should be the root partition and let it install.
Links:http://www.gentoo.org/doc/en/handbook/h ... stall-auto
Hope this helps.