Hi Paul ,
well...............I am still a firm believer in Yamaha ( although they are begining to price them selves out of the market in the last year or so ).
The main thing to check when replacing a 2 stroke with a 4 stroke is the weight difference.
This is where Yamaha score points . Most of their 4 strokes are designed from the ground up and therefore weigh more or less the same as their 2 stroke sisters .
Some manufacturers ( eg Honda ) use converted car engines and so end up with a very heavy engine with a poor power to weight ratio.
I recently drove a 6 metre Geminii with a 130 Honda which I feel would have been much faster and economical if fitted with just a 90hp Honda ...Some times more is less !
Check out the weight of your current engine and then check out the weight ( and price ) of a Yamaha 40hp and 50 hp 4 stroke .
I would reckon that a 40 hp would be oodles of power for a 4 meter RIB .
Best wishes ,