The advantages of a 4 stroke are said to be:
better fuel economy,
quieter running,
no oil mixing.
But, I usually don't use more than 15 gallons of gas a season.
I don't run the engine enough for those considerations to really matter to me.
I wish everyone had a 4 stroke, since the cumulative effect of all that fuel dumped in the water is disturbing. But, I'm ashamed to say, the environmental considerations would not tilt the decision for me.
As a practical matter, you've already heard that some engines will not fit in the motor well.
Equally important is the configuration of the throttle arm. If the throttle arm doesn't fit, you can't close the hatch cover. Also, you may not be able to swivel the engine 180 degrees. Generally, a throttle arm that tilts backwards will work better than one that tilts forward.
Best advice is to buy the outboard from a dealer with a good return policy. The sucker has got to fit.