code: public static <T1, T2, T3, T4> Func4<T1, T2, T3, T4, Void> toFunc(final Action4<T1, T2, T3, T4> action) {
          return toFunc(action, (Void) null);
      }
docs: Converts an {@link Action4} to a function that calls the action and returns {@code null}.
  
  @param action the {@link Action4} to convert
  @return a {@link Func4} that calls {@code action} and returns {@code null}