How to set new addresses on a multifamily??

[Sylvia Santelli]

Anyone have experience doing this? 

I just purchased a 4-plex, but the previous owner does not have separate addresses for each unit.

For example, I pushed 123 MainStreet and I want each unit to be represented separately, 123 MainStreet Unit A.

Do I need to do anything official with the post office or city? Or do I just put the mailboxes and numbers on each unit door?