2.c？
3.b？
4.( 1) Later, Li Zhen was found guilty and fled to the south. Zheng Ao walked thousands of miles to visit him. From then on, people who heard about it respected his virtue more.
(2) Zheng Ao once investigated this matter. If it is true, he did not explore this kind of Taoism (for Li Daoyin).
(3) Gaozu respected his words (or thought his words were very important), invited him into the inner hall to teach the Tao Te Ching and worshipped him as a teacher. (That's right)