I have an opportunity to assume the mortgage on a 21 unit portfolio at 4.1% 100% occupied. What questions do I need to ask beyond rent roll/ financials?

How did you act on this deal? In general, I would treat this like acquiring a 21 unit multifamily property. You really ought to run diligence on each building separately. Find out how long tenants have been at each property (you want to be sure the owner didn't just throw in "anyone") to get the max value on sale. I also recommend reviewing the leases and the details of the loan you plan to assume. The other thing that comes to the top of my mind in researching CAPEX or finding when the last significant repairs were made and getting proof.

I didn't act on it.  The current vacancy was too high for me.  Owner wasn't budging on the price. It's still listed for sale.  Thanks for the response.