In Yona 4:4 why does God need to explain/justify what he did to Yona?
God isn't justifying himself to Jonah, he is justifying himself to us, thousands of years after the fact. We read the story of Jonah on Yom Kippur afternoon to remind us of the importance of repentance. Note that the people of Nineveh do not convert to Judaism, they merely cease their evil ways.
Jonah is one of the literary prophets, the prophets that wrote down their messages in books named after themselves.