Where do I start first? First and foremost, the A4 is very reliable, extremely efficient, easy to work on, rugged, and cost efficient. When it comes time for total replacement, you could install three A4s for the cost of a similar HP diesel.
As for diesel being more fuel efficient than gasoline - not when you compare HP to HP. I'm pushing a broad-beam, 15,000-pound plus Morgan 33 O.I. with a 30-HP A4 gasoline. I get about 9 MPG at 1,800 RPM and cruise at 6 MPH on calm water. I have a friend with a similar boat, a Morgan 321, powered with a 3GM30 Yanmar Diesel. The boat weighs less, has a narrower beam and his fuel efficiency and speed are not as good as my A4. And, when it comes to repairs, the cost of replacing that Yanmar is out of this world.
Some folks believe that gas engines are all dangerous because of the possibility of explosion. If 6 decades of being on the water, and having owned 17 boats, spending an average of 70 or more days every year on the water, I have yet to PERSONALLY witness a single boat fire directly attributable to a gasoline engine - not one. I've seen two dozen fires directly related to electrical short circuits, and two diesel explosions. And, none of those fires were aboard sailboats - not a single one.
If you can find a good boat at the price you want to pay, I wouldn't worry whether or not it had an A4 or a diesel engine, as long as it the engine has been well maintained and in good condition.