Tax implication on buildings over 4-units

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Greetings,

one of the agents I work with in mid-Hudson valley (Putnam and Dutchess) keeps saying "well, because it's over 4 units there are tax implications..."

On the question "can you elaborate", I got an answer "well you are very knowledgeable I am sure you already know."

Anyone has any idea what is this tax implication is? 

Thanks