When I add all FlexUnit .swc, no error occurs. Add this to your build.xml :
<library-path dir="${FLEXUNIT_HOME}" append="true">
<include name="**/target/*.swc"/>
</library-path>
instead
<library-path dir="${FLEXUNIT_HOME}/FlexUnit4/target/" append="true">
<include name="*.swc"/>
</library-path>