Refer debian, it shipped the latest DocBook 4.5 XML DTD, as well as
a selected set of legacy DTDs for use with older documents, including
4.0, 4.1.2, 4.2, 4.3 and 4.4.
About the config files (docbook-xml.xml and the update patch), we
refered what Ubuntu 13.04 did, so the xmlto could correctly search
among multible DTDs.