version = "unknown"
when it should say:
version = "1.2"
Therefore hg on el4 reports a bad version number which makes it impossible for other software to support multiple versions of hg (as they can not use the correct API which changed between hg versions). For the example the trac mercurial plugin (not packaged in Fedora) will cause exceptions in certain situations (see http://trac.edgewall.org/ticket/9611#comment:5).
I confirmed that manually setting the correct version in __version__.py fixes the problem. To me it looks like some missing buildrequires.
EPEL-4 has reached end of life and is no longer supported.
Please retest your bug against EPEL-5 or EPEL-6 and re-open if the bug persists in the packages available in those releases, or file a new bug.