LOL... Yea, the sweater isn't needed. The battery is completely enclosed by the body and there's a layer of still air around it which provides a degree of insulation.
I've done flying tests down to -33C (-27F) paying particular attention to battery temperatures. I have a record of the temperatures at take off and landing through various outdoor conditions.
If the battery is at room temperature, it will get warmer after you take off and will continue warming during the flight. After about 15 minutes, the battery temperature will be up around 38C (100F) and it will remain there until you land.
My battery temperature with an outdoor temp of -30C (-22F) only got up to 29C (84F). But, despite that I experienced no reduction in flight time. The battery is quite happy operating at 84 degrees F.
With outdoor temps below -30C (-22) I found that you had to work the craft in order to generate enough current to keep the battery warm. The battery temperatures did warm continuously after take off and flying with the stick fully forward. But, if I went into a hover (which uses less current), the battery temps did start to fall by about a degree per minute.
So by my experience, the P4 will fly just fine down to -30C (-22F). Below that, perhaps one of those sweaters might just make a difference
