I just built a 4*60 & put a Saito .91FS in it. I absolutely love the plane & the engine. This thing is so much more fun to fly than my trainer. The Saito sounds great & it has more than enough power. I have an OS .61FX in my trainer, but I don't think it would be near as much fun in the 4*.
I use Byron's 15% nitro & 16% oil. This works very well for me. There is a small % of Caster oil in it which gives me piece of mind. It stains the muffler a little, but I just clean it with Z-Best every now & then.