Your question isn't clear. Is the problem that you can't choose CD-ROM as a boot option, or that when you choose it, your system just boots from the hard drive as normal? If it's the latter, the most likely cause is that your FC4 CD wasn't burned properly.
Check that the md5 checksum for the downloaded ISO is valid, and make sure your burn the ISO as an image, not a regular file (dragging and dropping won't work, you'll need to choose the "Burn as Image" option in your app). If the md5sum doesn't check out, then the image is corrupt and you'll need to download it again. Good luck with it