The current cap rate is 3.75% for 1.9M. for the buyer ROI may work out 3.75+6% = 9.6% range going forward.
For the seller, the current cap rate is calculated on his investment amount 720k. Then 2017 seller cap rate is 9.76%. If we rollback rent every year 3% down, his 2000 year cap rate is 7.5% (appx deducted 21%).
For the seller, ROI works 7.5%+6% = 13.5% which is great.
IMO, mostly this sale may be 1031 exchange or step up basis inheritance sale. Other than this, I really do not see any point in selling this 4-plex.