Having gone through this with my last boat - a 36 footer - I formed several opinions based on the experience.
1. Unless I had the motor permanently mounted to the dinghy - that is, I didn't need to put it on/off every time I put the dinghy in the water - I would never buy another 4 stroke. They are too heavy.
2. Remember that the motor capacity is primarily based on the weight of the motor. Therefore, if your 4 person dinghy says, for example, that it will accept a 10HP motor, it is based on the weight of a 2 stroke outboard. Putting a 10hp 4 stroke motor on the boat means the bow will ride so high as to be a problem.
3. Bigger HP motors on small dinghys are essentially pointless. Our current 11ft Avon has an old 4HP merc on it and it functions just fine. I also previously learned that being able to plane the dinghy is of no value at all. It just beats you to death and is squirrelly as heck. Sure, it's faster to plane but really, how quickly to you have to get the mile or two that you might need to go?
4. As per a previous note, I would set up the dinghy in some manner that did not require installing/removing the motor. It is an accident looking for a place to happen and you WILL drop the motor at some point. It may go in the water or it may bang the heck out of the deck/swim platform, but you will drop it. Again - lighter is better.
I bought and sold the new 4 stroke outboard within 3 months. I vowed never to buy another 4 stroke small outboard. The only advantage - not mixing the oil- is inconsequential compared with the weight disadvantage that affects several aspects of use. Certainly if the same HP motor weighed the same as a 2 or 4 stroke, I would buy the 4. But they don't.
I've heard there are now some areas where 2 strokes can't be used at all. Obviously, in that case, you have no choice but to buy a 4. In that situation I would look at the HP rating of the dinghy, determine the weight of the appropriate 2 stroke motor, and buy the 4 stroke of that or less weight - whatever the HP rating happened to be (it will be less).