It seems that in certain cases Prince ignores negative margin values. I prepared small test case that illustrates possible bug (test page is handled properly by Opera and Mozilla):
http://geocities.com/csssite/prince/bug07.xhtml

Thanks, it appears that negative values are not handled well for the margin-bottom property; we will investigate this issue for the next release.

For the record, this bug has been fixed in Prince 5.0.

Thank you for reporting the issue!