The 4 in the 4-60 stands for four stroke. This model was designed to fly on those old, powerless four stroke, manufactured in the late 80s. They fly just fine with them, too.

I've built two 4-60s. Both were overpowered and flew best at reduced throttle settings. One had an OS 91 Surpass. The other an OS 61

I flew on powered by an OS 61FS. It was the right power for the design.

My suggestion would be the 70 Surpass.