RE: AX Engines
You shouldn't have to worry about an engine not lasting 10 gallons for your RV-4. Most RV-4s don't get that much flight time anyway. It's a sport plane, fly around type of plane. People who own RV4 usually don't fly it as much after a few year or so. 1 gallon of fuel will provide 12~16 flights, which should last a month if you fly on the weekend, 4 flights per weekend. Assuming you fly nothing else throughtout the year, you might hit the 10 gallon mark after a year.
I would actually recommend Thunder Tiger Pro 46, the undisputed champion of .46 2-stroke engine in terms of reliability. I have one. Even after getting run over by a car (I'm serious), the engine still runs like a champ, swinging MA 9x6 @ 17000 rpm or APC 12x4 @ 14000 rpm. This is an OS in its former glory before things went south in some of OS products.