I got to ask that as well because he was flying with the 4. Alan's 5 weighs around 57 lb and has an external gas tank, and according to the chart, the 4 with the same 15" shaft weighs the same. It holds .29 gallons where the external is 3.1.
Some thing to look for on smaller motors is "reverse" and "idle"
Advantages to a gas are, the fact that the motor weighs the same as a battery. You can go allot farther and faster with a gas.
Disadvantage, when you do turn it off, you have to pull a cord to re-start, and if you want to troll, gas goes way to fast.
Cost is another thing. You will be looking at around 1K and higher, but won't need a charger after each trip.