I understand. I *do* notice that the package I mentioned (ac3dec) has been replaced by a newer (a52dec) package which does exist for el4. I was just curious because I have encountered sort of "interesting" packages that I might want to try on dag (eg. multi-gnome-terminal) and are not built for el4. I will keep what you say in mind. However, if I have questions, I'll probably post on the rpmforge mailing list. :-) Thanks again!
I don't think I fully addressed the question before, so allow me to clarify. EL3 binaries are linked against different versions of libraries than are in EL4, and in some cases library names have changed. Glibc and kernel differences may also cause issues. If you want to install an EL3 package, I'd rebuild the srpm at a minimum, but if dag suddenly stopped building it or releasing it for a particular EL version, he may have a good reason for this. It may be advisable to ask why it was not built/released for EL4.
Hope that more clearly answers the question.