At the moment FUC is

package TWiki::Contrib::FuncUsersContrib

this is correct, but it needs to change to also extend TWiki::Func, thus

package TWiki::Contrib::FuncUsersContrib;

# VERSION and suchlike stuff
# stubs linking to TWiki::Func methods

package TWiki::Func;

# actual impls

That way plugins that use it can work in TWiki-4 by:

if (this_is_twiki4) {
   eval 'use TWiki::Contrib::FuncUsersContrib';
}

CC