"Depends on your application.
"Depends on your application. The 9.9 4's will burn less fuel, so if hauling fuel or deliveries of fuel are pricey...then 4's have that advantage. You will have to have enough motor oil on hand to service the 4's for a season of running. But you won't have to buy any of the pricey 2 stroke oils. If top end speed is not critical and low fumes/emmisions are..then 4 strokes win again. Long hrs of trolling would favor the 4's. The 4's take longer to warm up at the beginning of the day, fuel must be kept cleaner for 4's (I would add a good filter if you don't run one now with the 2's) The 4's are easier to trade off when your done with them. (more demand) Lastly..the largest outfitter in Canada (Oak Bay Marine Group) use 4 stroke Yami's."