Searching Packages:
Setting up repositories
not using ftp, http[s], or file for repos, skipping – 4 is not a valid release or hasnt been released yet
Cannot find a valid baseurl for repo: update
Error: Cannot find a valid baseurl for repo: update

End-of-Life (EOL) for CentOS 4 proved to be the root of the problem. The current CentOS version cannot be upgraded at this time, and I expect that others will experience the same difficulty. There is a way around this problem.

Open /etc/yum.repos.d/CentOS-Base.repo in an editor
  Comment all “mirrorlist” entries.
  Uncomment the “baseurl” entries and change them to http://vault.centos.org/4.9/os/$basearch

Then yum will be able to access the repository of CentOS rpms.