dwrap4<R> method
Binds a cwraped C function taking 4 arguments and returning a plain
(non-struct) value.
R determines the JS->Dart return conversion applied on every call
(see _conv); use void if the C function returns nothing.
Implementation
WF4<R> dwrap4<R>(String name) {
final f = _wn(name, 4);
final c = _conv<R>();
return (a1, a2, a3, a4) => c(f.run([a1, a2, a3, a4]));
}