If your current motor is running great why are you looking to spend 14-15 k to put a 4 stroke on of the same size?
If it's for the fuel economy you will have do be doing a hell of a lot of boating to get your money back.
The 4 stroke might also be significantly heavier then your current motor.
All moden outboards are good quality. But make sure you find a good dealer that is easily accessible and will back you if you have issues.