Add a Mailbox to a multifamily?

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Hi BP Community,

Hoping someone can help me. I tried usps but there's no clear option to choose for this. I have a legal 4 family unit. There are only 3 mailboxes. the 4th unit is a basement unit but i've confirmed with the town and the state that my building is a legal 4. How would i go about adding a 4th mailbox? 

The current 3 mailboxes are set up in the entrance area recessed into the wall and protruding into my first floor unit's closet. The basement has a separate entrance. Preferably, i wouldn't replace the existing 3 mailboxes, and i'd just add another mailbox by the basement entrance. 

Anyone know what steps I need to take to do this? If i get a mailbox without a lock, can i just leave a note at my other mailboxes asking the mailman to please drop off the basement unit's mail at the basement unit entrance?

thanks!