第一种方法:
yum源来自chinalinuxpub.com,用的是centos包,
下载地址:  http://www.chinalinuxpub.com/yum.tgz
安装:
tar zxvf yum.tgz
cd yum
rpm -ivh *.rpm

rm -rf /etc/yum.repos.d/CentOS-Base.repo
vi /etc/yum.repos.d/as4-update.repo
加入：
#released updates
[update]
name=Red Hat Enterprise AS release 4 Updates
baseurl=http://ftp.chinalinuxpub.com/redhat/enterprise/updates/4AS/
gpgcheck=1
gpgkey=http://ftp.chinalinuxpub.com/redhat/enterprise/RPM-GPG-KEY-linuxpub

vi /etc/yum.repos.d/as4-base.repo
加入：
#released base
[base]
name=Red Hat Enterprise AS release 4 Updates
baseurl=http://ftp.chinalinuxpub.com/redhat/enterprise/os/i***/RedHat/
gpgcheck=1
gpgkey=http://ftp.chinalinuxpub.com/redhat/enterprise/RPM-GPG-KEY-linuxpub

然后就可以正常使用yum了.

第二种方法:利用CentOS 的yum库升级RHEL AS4
1.下载并安装yum-2.4.0-1.centos4.noarch.rpm文件，下载地址为：
ftp://ftp.pbone.net/mirror/ftp.centos.org/4.2/os/alpha/CentOS/RPMS/yum-2.4.0-1.centos4.noarch.rpm

2.修改或建立/etc/yum.repos.d/CentOS-Base.repo为如下内容：
[base]
name=CentOS-$releasever - Base
baseurl=http://ftp.riken.jp/Linux/caos/centos/4.0/os/$basearch/
gpgcheck=1

#released updates
[update]
name=CentOS-$releasever - Updates
baseurl=http://ftp.riken.jp/Linux/caos/centos/4.0/updates/$basearch/
gpgcheck=1

#packages used/produced in the build but not released
[addons]
name=CentOS-$releasever - Addons
baseurl=http://ftp.riken.jp/Linux/caos/centos/4.0/addons/$basearch/
gpgcheck=1

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
baseurl=http://ftp.riken.jp/Linux/caos/centos/4.0/extras/$basearch/
gpgcheck=1

3.修改或建立/etc/yum.repos.d/dag.repo为如下内容：
[dag]
name=Dag RPM Repository for RHEL4
baseurl=http://ftp.riken.jp/Linux/dag/redhat/el4/en/$basearch/dag/
enabled=1
gpgcheck=1
gpgkey=http://ftp.riken.jp/Linux/dag/packages/RPM-GPG-KEY.dag.txt

4.运行update:
yum update

5.升级：
yum upgrade

6.安装其它软件，例如：
yum install mplayer