> /etc/yum.repos.d/CentOS-Base.repo has this:
>
> #released updates
> [updates]
> name=CentOS-$releasever - Updates
> mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
> #baseurl=http://mirror.centos.org/centos/$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&repo=updates
> baseurl=http://www.muug.mb.ca/pub/centos/5.4/updates/i386/