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If you're about to close your biz card account, log into your Citi biz account online, then look at the drop down for accounts. One of those will have the last 4 digits of your biz card; that's your card account. The other will have different set of last 4 digits which don't match your biz card; that's your master account. Now that you have the last 4 digits of both: Send a secure message to Citi asking them to close both the card account ending in CDEF and the master account ending in MNOP.

If you recently closed your card account, and you can still log in and see it online, then use a variation of the above technique, where you send a secured message asking only to close the master account (giving its last 4 digits), since the card account is already closed.

But if you closed your card account so long ago that you can no longer log in and see it, I don't know how you find out your master account last 4 digits, and thus I don't know how you explain to Citi what exact account you want to close.