4-Family with Semi-illegal Basement Risky?

[Matt Liu]

I am about to buy a 4-family for what seems like a very good deal. The issue is that the city of Jersey City apparently has is down as a 4-family, while the state of NJ has it as a 5-family. The basement unit does indeed have a tenant and even a sperate electric meter.
I'm wondering if there is a risk of being fined or even sued later on if it is discovered by the city? If the risk is minimal, it may be worth taking, because that basement unit brings another 500/month and I have not rent control due to the city calling it a 4-unit.