You may have to recompile G4Java. It consists of a Java part and a native C++ part. The NAR Plugin handles the compilation of the native part and its native dependencies (geant and clhep).
To download G4Java type
svn co svn://svn.freehep.org/svn/sandbox/G4Java
and compile it by typing
This may take quite some time the first time as the correct geant and clhep libraries need to be downloaded and unpacked.