yes you can use f4 but you'll not be able to get all the fine details when they look like a veil (sorry English not my language too) by using a very short duration (1 to 3sec), also you can get pin point stars and that (IMHO) more beautifull.Aurora I wouldn't call as Astro... (yes yes, Sun...) - But, yes, for Aurora a fast lens is very good. (But I did it also with f/4... works).
But is this the target for the 14 IS here? A special lens for all Aurora enthusiasts? This "big" market?
same thing for Milky Way, when you don't want to use a tracker (or if you don't have), it's a lot easier with big aperture to get pin point stars
last but not least, about IS : I try to travel to make pics and weight is limited, so I also use this lens for landscape, so if it has IS it's better for me.