Heian means peaceful, as in a peaceful mind.
The idea is if you iknow the applications, you have a peaceful mind as to the understanding of basic but robust self defense - armbars, throws, escapes, takedowns are all the Heians. Fits in with the idea of having a basic martial competency before reaching Ydansha.
The original name was Pinan. This simply meant belonging to a child/children. They were, children's kata, with some applications specifically for smaller people, or the Kanku/Channan kata split up, (as they are long and difficult).
Yondan simply means fourth, as you can guess.