I don't know about the OS 4 strokes except they're pretty heavy for the power output. I have a Saito 82 on my UCD 40 and it's great. It hovers with ease and I've never felt a lack of power. Pull out is good enough to be comfortable at 5 to 10ft agl. A friend has a Saito 100 on his and he has much better verticle but that's the only benefit. For my 100 I chose a Twist 60.