code: protected boolean is4YearTerm(Term<?> term) {
  
          if(!(term instanceof DateTerm.YearTerm)) {
              return false;
          }
  
          DateTerm.YearTerm yearTerm = (DateTerm.YearTerm)term;
          if(yearTerm.getFormat().length() == 4) {
              return true;
          }
  
          return false;
  
      }
docs: 項が日付の4桁の年かどうか
  @param term
  @return