H4 files with H-1B transfer?

My current H-1B expires on the last day of 6-year H-1B limit, and my dependents have the same expiration date for their H4. If I change employer, the H-1B transfer will keep the same expiration date.
Do my dependents have to file for new H4 with my H-1B trasnfer?
My understanding is they may not need to file since H4 status is not employer dependent. Their H4 is valid as long as my H-1B is valid. But I am not sure. Please provide some info and advice. Thanks.