[BrianG]

I think it's all about the gearing, within reason.

You can run a 35XL (if it existed) on 4s, but it won't work well if geared too high to compensate for lack of rpm because it is mechanically loaded too much. Plus, turning these motors too slow is bad because the ESC will have a harder time to get the rotor position when starting.

If you run too high a voltage to a motor, it will spin too fast and overheat just from sheer rpms regardless of a light mechanical load.

That's why I like to choose a motor wind that will give me around 30-35k rpm (for Feigao and similar) or 40-45k (for Neu and similar) with the voltage being used. It's a good balance between a high, yet safe, max rpm and allows reasonable gearing for a proper meachanical load.

For 4s, I would go with an 8XL Feigao. If you think you might go with more voltage (an extra lipo cell) later on, go with a 9XL for a little wiggle room. Just my $0.02.

By the way, you can see my Hyper8 buggy in the BL Conversions link in my sig (scroll down a bit). I was running 4s on a Neu 1512 2d (2600kv) and everything ran very well even geared high at 46/16. I've since dropped a couple pinion teeth and the ESC/batt/motor is barely warm (~110*F or less depending on ambient temp).