If you run a yum update on a CentOS 4 Linux server, you may encounter the error “not using ftp, http[s], or file for repos, skipping – 4 is not a valid release or hasnt been released yet”.
This is because CentOS 4 has been end of life’d. There is a pretty straight forward workaround to resolve this problem.
1. vi /etc/yum.repos.d/CentOS-Base.repo
2. Comment out the “mirrorlist” entries.
3. Uncomment the “baseurl” entries and change them to http://vault.centos.org/4.9/os/$basearch