1. "I don’t speak Japanese” (plain form)
2. “I don’t speak Japanese” (polite form)
3. I’m not sure, but this sentence seems odd without knowing the context. Maybe in English it would be "I can’t understand (it) in Japanese”. There is an invisible reference here in the Japanese (rendered by the word “it” in English) to a previously mentioned object.
4. If this is a question, it should be “anata wa yasahii hito desu ka?". This means "Are you a kind person?"