How to Install RHEL 4 or CentOS 4 Linux
Source: http://www.linuxmail.info/how-to-install-centos-4-linux
After getting a copy of the CentOS 4 Linux or Red Hat Enterprise Linux 4, the steps below describes how to install it from scratch.
1. Boot your computer from the CentOS 4 CD 1 and press Enter on the CentOS boot up screen. This will install CentOS in graphical mode.
2. Choose Skip in the CD Found window. This will skip the CD media test.
3. Click Next in the Welcome to CentOS-4 i386 window.
4. Choose a language in the Language Selection window and click Next.
5. Choose a keyboard in the Keyboard Configuration window and click Next.
6. Choose Server in the Installation Type window and click Next.
7. Click Next in the Disk Partitioning Setup window and answer Yes on the Warning window.
8. Click Next in the Automatic Partitioning window and answer Yes on the Warning window.
9. Click Next in the Disk Setup window.
The automatically generated partitions works fine but you can fine tune it from this window. Learn more about Linux partitions here.
10. Click Next in the Boot Loader Configuration window.
11. Configure your network setting in the Network Configuration window.
Let’s say you have a network settings like the one below
Host Name
mail.acme.local
IP Address
192.168.0.4
Subnet Mask
255.255.255.0
Gateway
192.168.0.1
Primary DNS
192.168.0.1
Secondary DNS
192.168.0.2
Tertiary DNS
192.168.0.3
First, click the Edit button. In the Edit Interface eth0 window, uncheck the Configure using DHCP and fill up the IP Address and Netmask. Click OK when you’re done.
Next fill in the rest of the information in the Network Configuration window. Click Next when you’re done.
12. Set Enable SELinux? to Disabled and click Next on the Firewall Configuration window.
It is important to disable Security Enhanced Linux. Setting it to disabled does not make your system any less secure, but it will make your system more predictable. Otherwise, you might find your self spending hours fixing problems caused by it. Here’s one example from the Red Hat Knowledgebase.
13. Choose the default language for your system and add any other languages in the Additional Language Support window. Click Next when you’re done.
14. Choose a time zone in the Time Zone Selection window and click Next when you’re done.
15. In the Set Root Password window, specify a strong password at least six characters long and click Next when you’re done.
16. In the Package Group Selection window, check the following packages to add it to the installation
X Window System 
Gnome Desktop Environment 
Graphical Internet 
Mail Server 
Development Tools
Click Details corresponding to X Window System and check system-switch-mail-gnome.
Click Details corresponding to Mail Server and check postfix and squirrelmail
17. Click Next in the About to Install window. It will show you the CDs required, click Continue to proceed. This will start the installation process.
18. Congratulations, the installation is complete. Remove the CD and click Reboot to proceed to configuration.