Only 4km? That distance really most decent bikes should handle fine. I agree with belt drive. Maybe even single speed to cut down on parts that could go wrong in the cold. Front and rear drum brakes. Might want to look into handlebar hand protectors, which block the wind off your hands. I'd also suggest steel fenders and a frame without a lot of odd shaped tubing, as who knows how the cold will affect non-round shapes. I also suggest kenda tough tubes, which are thicker than regular tubes.

- Andy