One of PGR4's biggest new features is an advanced dynamic weather system - can you elaborate just how much this will affect races?

Sure can! We have quite a few different types of weather in the game - rain, snow, fog, ice, storms, etc. - and all of these affect your vehicles dynamics in different ways. For example, driving on black ice is obviously much more treacherous than driving in clear conditions as you'd expect. As the weather is dynamic, the race might start in gentle snow but then progress to a full-blown blizzard by the third lap, or any combination like this.