To repower solely for the cost of fuel would be a mistake in my opinion. If there are other reasons like quiet motor, reliability, no smoke etc then fine but just to save fuel makes no sense.
For example - to put new Yamaha 200 4 strokes on my Dolphin would run between 33-35k. Lets say I can sell my existing Yamaha 200 2 strokes for $6000 for the pair (purely a guess there)- That means I am laying down 27-29K for new power. If solely for gas I would have to use one heck of alot of gas to make up that difference. Now of course my resale value would go up - I guess we could figure that in - but still I think it would take a really long time to recover that cost -
If you find a nice used pair of four strokes that you can get a nice deal on then maybe the math would change.
Just my 2 cents - of course opinions vary on this.