Cent 4.9 is End of Life on Fed 2012.
Inorder to do an update (for the last time, you need to):

1. Edit your /etc/yum.repos.d/CentOS-Base.repo, replace the line like
   > baseurl=http://vault.centos.org/4.9/os/$basearch/
   ---
   < mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
   < #baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/

The files have been removed from the mirror, and put into a 'vault' area.
Here is a filed used on t2h:

      # CentOS-Base.repo
      #
      # modified  CentOS-Base.repo to refer to http://vault.centos.org/ rathera that http://mirror.centos.org
      #
      #

      [base]
      name=CentOS-$releasever - Base
      baseurl=http://vault.centos.org/4.9/os/$basearch/
      gpgcheck=1
      gpgkey=http://vault.centos.org/4.9/RPM-GPG-KEY-centos4
      priority=1
      protect=1

      #released updates 
      [update]
      name=CentOS-$releasever - Updates
      baseurl=http://vault.centos.org/4.9/updates/$basearch/
      gpgcheck=1
      gpgkey=http://vault.centos.org/4.9/RPM-GPG-KEY-centos4
      priority=1
      protect=1

      #packages used/produced in the build but not released
      [addons]
      name=CentOS-$releasever - Addons
      baseurl=http://vault.centos.org/4.9/addons/$basearch/
      gpgcheck=1
      gpgkey=http://vault.centos.org/4.9/RPM-GPG-KEY-centos4
      priority=1
      protect=1

      #additional packages that may be useful
      [extras]
      name=CentOS-$releasever - Extras
      baseurl=http://vault.centos.org/4.9/extras/$basearch/
      gpgcheck=1
      gpgkey=http://vault.centos.org/4.9/RPM-GPG-KEY-centos4
      priority=1
      protect=1

      #additional packages that extend functionality of existing packages
      [centosplus]
      name=CentOS-$releasever - Plus
      baseurl=http://vault.centos.org/4.9/centosplus/$basearch/
      gpgcheck=1
      enabled=0
      gpgkey=http://vault.centos.org/4.9/RPM-GPG-KEY-centos4
      priority=2
      protect=1

      #contrib - packages by Centos Users
      [contrib]
      name=CentOS-$releasever - Contrib
      baseurl=http://vault.centos.org/4.9/contrib/$basearch/
      gpgcheck=1
      enabled=0
      gpgkey=http://vault.centos.org/4.9/RPM-GPG-KEY-centos4
      priority=2
      protect=1

Now you can run yum update.