/etc/yum.repos.d/CentOS-Base.repo has this:
#released updates [updates] name=CentOS-$releasever - Updates mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&rep... #baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/ gpgcheck=1 enabled=1 priority=1 gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
You can update this yourself to hardcode the 5.4 version of your mirror, e.g someone else in this thread said this was their mirror :
http://www.muug.mb.ca/pub/centos/5.3/updates/i386/
and you can change the 5.3 to 5.4 to see that 5.4 is there already, but maybe the "5" symlink has not yet been changed to point to it
http://www.muug.mb.ca/pub/centos/5.4/updates/i386/
So update the repo file to something like this :
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&rep... baseurl=http://www.muug.mb.ca/pub/centos/5.4/updates/i386/