Yamaha would be my choice also. One thing to keep in mind is that the 4-stroke is going to weigh considerably more. I had a 15 1/2 ft aluminum smokercraft with a 25hp 2-stroke Yamaha and it was perfectly matched. I later sold that motor and bought the 25hp 4-stroke Yami. It was ok, but the boat handled considerably different due to the extra weight back on the transom.
On anothher note, when I first bought the boat it had a 9.5 Evenrude on it. It worked out just fine with 2 people in the boat, but as soon as I added a third person or additional weight it was extremely hard getting it up on a plane. In a nutshell, my advice would be to re-think the size motor your going to purchase and consider getting a 15-25hp motor. Then consider the additional weight of the 4-stroke.