RE: Castle Creations
They are obliged, as a manufacturer, to cover their own butts! If they say to you its ok to use a 4000mAh 15C battery and then your battery melts (because your setup pulled more than 60A), or your ESC fails, you can't blame them. It's not that it's 'required', it's what's 'recommended'.
As far as using batteries with lower capablities, it depends on the motor. If you know the motor current will never exceed 60A continuous and 90A burst, then you can safely use a 4000mAh 15C battery. The battery won't last as long as a higher specced one would, and may not give as much punch, but it shouldn't cause a failure.