Description of problem: 4000MB is too small to support yum upgrades etc so I would suggest increasing the default disk file size to about 5000MB.
That is entirely dependant on what set of packages you have installed. I can easily demonstrate both that 5 GB is too small for some package sets, and that 4 GB is overly large. It is also dependant on what architecture (eg x86_64 is multilib), what OS version (some versions of YUM are more efficient than others), and even what OS (Windows doesn't use YUM). It is non-sensical to set defaults based on the hypothetical requirements on a particular type of install of any single OS.
Erm, granted, but then why 4GB? :) (I am basing on default Fedora i386 installs FWIW.)
Another datapoint: current rawhide livecd-desktop image won't install with 5GB even.