Beyond the obvious, what are you planning to use the boat for?
If all you plan to do is idle back and forth from your boat to the dinghy dock
then hp is of little consequence.
If you want the boat to reach its safe and efficient top speed with the rated max load then you need to read the plate on the transom and see first what is suggested. If you have the LSI 290 they suggest 8 hp max. And 6hp as standard. I think 4 might be far too little when the boat has a load. When a soft floor inflatable
bends as often happens when its near the max load and or the air pressure is low, it will take even more HP to push it along. Thats a considereation also.
I have a 9.9 on a 10 foot avon
RHIB and the boat is just slightly underpowered with 4 adults. When its just me and some groceries its more power than I need. That boat is rated for 15hp so I am just about at half the recommended power.
When we had a catastrophic transmission
meltdown on the ICW
I put the RHIB along side and warped my 44 foot sailboat over 15 miles. This saved a huge tow bill and worked marvelously.
More HP that day would have helped somewhat but less would have certainly been a problem.
How will the motor
be stored? If it has to go on and off the boat everytime you use consider the handling gear
you will need. More HP equals more weight.
Will it spend most of its time hanging on the rail? Thats a consideration also. Will you have to beef up your rail to hold a larger outboard