During the closing process my realtor had notified the seller's realtor to give the current tenants notice that they had to leave. 

If a bank is telling you that you cannot get traditional financing because it is a duplex, you definitely need to leave that bank.   

I didn’t want to over pay and leave myself no room for the Brrrr

Should I just leave it until the lease is up?

my main concern Is Oregon inheritance tax... :)  

I will soon have a $100k downpayment going into new primary home which will still leave me with $200k cash that I can use as capital. 

I don't mind leaving all positive cash flow in this 4 plex for repairs for the next several years. 

I am intending to use an FHA loan (or FHA 203K loan depending on the nature of the deal) to fund the purchase of either a one or two unit piece of real estate.However, I will be leaving my job soon to pursue an entrepreneurial opportunity.

Anyway, my mother-in-law mentioned over dinner a while ago that they might be moving in the next few years leaving their nice 5 bedroom 3.5 bathroom house in Broomfield, Colorado vacant!

The sellers have already asked the current tenant to leave.