Can the credit be accumulated on my Nespresso account? E.g. if not used for 4 months will the amount of credit be carried to the next month's plan.

Yes. Club Credits can be accumulated over 24 months to use whenever you want. You do not have to use up your credit every month. In addition, if the credit is only partially used, the remaining will carry over. Club Credits will even be valid for up to 12-months after your subscription ends. After that we will convert any unused credit into capsules or accessories for you to use.