With 4 premium accounts you would be paying the same as with a families organization. However, with a families organization you can share multiple collections with all the other members of the org (each collection with its own permissions).
With 4 premium accounts your sharing options are limited to a single organization with 2 members and 2 collections maximum.
And the items in that organization vault do not have any premium features (for example: they can not have attachments and you can not use the TOTP functionality with them).