1. He is known to us.
2. He is known for novels.
3. He is known as a writed.
4. A man is known by the friends he keeps.
(Are they all grammatical? Am I using prepositions well?)
All the prepositions are used correctly, yes. Here are a couple of corrections for you, though:
2. He is know for his novels.
3. He is known as a writer.
=)