If you already have a Drivemob account nothing will change with your subscription when you upgrade to a new Softbox 4. Your drivemob subscription will work on the new box and stay active.
You will just need to login to your account to look up your username and password. You will need this info to login in to your Softbox 4. To look this information up, go here to see how: https://intercom.help/softbox/en/articles/12003573-log-in-to-online-account
If you don't have a Drivemob account. You will need to create one when you follow the set up instructions to your Softbox 4.