5-Unit Listed as a 4-Unit - Is this cause for concern?

[William Reed]

I am considering purchasing a multi-family property in Northern New York.  It's listed at $103k as a 4-Unit with an office, however the owners have the office rented as a 5th unit.  They told me that they don't report the 5th unit because the financing and taxes would have been greater because it would be considered a commercial property.  As a new investor, I'm concerned of the ethics of not reporting the 5th unit and assessing it properly but am not sure if I'm making too much of the situation.  I would imagine that a home inspection and appraisal would  uncover the fallacy.  Would it be best for me to finance it as a 5-unit or keep it as-is?  Personally, I would rather have a 5-unit because I can directly force the appreciation as opposed to a 4-unit that is dependent upon comps in the area.  

Would there be any ramifications on the current owners if it is assessed as a 5-unit during an appraisal?  The owners made it clear that their main goal with all of their investment properties was to avoid taxes (they've  informed us of some other practices where they don't report cash rent payments and income from a washer & dryer).  Ultimately, I want to be honest and transparent in all of my business practices - if it causes me to pay more in taxes and financing, so be it.  If anyone has any insight or advice, it would be greatly appreciated.

-Will