H4 duration is determined by your H1 approval notice

So if you are transfering, their H4 will only cover until your current H1 expiration date. Because they can only show the old I797.

It is better if you get your new H1 approval notice (i.e. new I797), then apply for H4. If you roughly know when it will be approved, you can let them go back first and mail them the I797 before they go for interview at consolate. Though personally I won't recommend it. Better they carry it with them.