Edited 19 August 2008
The problem with using Java 1.4 is that JAXB requires Java 1.5, and docx4j relies heavily on JAXB.
J2SE 1.4.2 is in its Java Technology End of Life (EOL) transition period. The EOL transition period began Dec, 11 2006 and will complete October 30th, 2008, when J2SE 1.4.2 will have reached its End of Service Life (EOSL).
That's next month. So its unlikely JAXB will ever officially support 1.4.
There are workarounds however - see http://forums.java.net/jive/message.jspa?messageID=212494
(regarding retroweaver or retrotranslator and JAXB).
See further http://www.nabble.com/Switching-to-Java-1.5-p17673100.html
(FOP discussion) and http://www.nabble.com/OOXML-branch-and-JDK-1.4-td19451507.html
By the way, docx4j SVN Head now assumes you're providing the JAXB implementation, rather than using the version provided with Java 6. If you were using Java 6, you'd need to put it in your java.endorsed.dirs.