You needn't do it again.
正确.need作情态动词用.
2.You don't need do it again.
错误.need在这里作实义动词用,应该是need to do.
改:You don't need to do it again.
3.We don‘t need to tell him the truth.
正确.need作实义动词.
4.We need't tell him the truth.
句子结构没问题,need作情态动词.就是needn't少打了字母.