I believe that S&W "standard blend" airplane fuel is 2% castor, 18% synthetic lube. This is perfectly suitable for a 4-stroke and for most 2-stroke engines. If your 2-stroke has plain bearings (bushed, not ball bearings) you might benefit from a higher castor content.
I was not aware that S&W produced a special fuel for 4-strokes. Call Stan (the owner) and ask.
www.s-whobby.com