differences between RC3 and RC4 install images

[Andre Robatino]

The install images for 20 Beta RC3 and RC4 are definitely NOT identical. The
checksums are different, and just looking at the DVD, the options under
"Installation source" are different (RC3 only had one option, RC4 has the
regular complement) and the add-ons for "Basic desktop" are different (RC3
had Security Lab, which normally isn't there, RC4 does not). There are
probably other differences as well, I didn't check thoroughly. It looks like
something was broken with RC3 install images and fixed with RC4.