It would be quite unusual to get 5-6 meter waves at BF4. That's more of a BF8 level. I bet it's misreported. Most ships, even small ones, will not care for BF4.
Couldn’t the waves originate elsewhere where it’s BF8? What’s going to stop a wave in the open ocean?
Other waves. The ocean is pretty disorderly
Waves can travel, but big waves quickly spend their energy, because, unlike swells, they break. When the wind goes down, waves quiet down very quickly.