I did a small basic FC4 install using anaconda, then started adding the stuff I
needed using yum. In a few cases, the installs would fail because of errors like:

Downloading Packages:
warning: rpmts_HdrFromFdno: Header V3 DSA signature: NOKEY, key ID db42a60e
public key not available for sgml-common-0.6.3-17.noarch.rpm
Retrieving GPG key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora

The GPG key at file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora (0x4F2A6FD2) 
is already installed but is not the correct key for this package.
Check that this is the correct key for the "Fedora Core 4 - i386 - Base" repository.


Doing 

# rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY

fixes the problem for the couple of packages I saw this on

The issue is that packages in Fedora Core are signed with (at least) two
different keys. Most are signed with /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora but
some are instead signed with /etc/pki/rpm-gpg/RPM-GPG-KEY

To fix,

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY

should be added to either /etc/yum.conf or /etc/yum.repos.d/fedora.repo to go
along with the 

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora

that's already there