The readme located on CD#1 /images directory seems to indicate that you can't use a floppy anymore. Probably too big of kernel.
This directory contains image files that can be used to create media
capable of starting the CentOS 4.0 installation process.
The boot.iso file is an ISO 9660 image of a bootable CD-ROM. It is useful
in cases where the CD-ROM installation method is not desired, but the
CD-ROM's boot speed would be an advantage.
To use this image file, burn the file onto CD-R (or CD-RW) media as you
The diskboot.img file is a VFAT filesystem image that can be written to a
USB pendrive or other bootable media larger than a floppy. Note that
booting via USB is dependent on your BIOS supporting this. It should
be written to the device using dd.