Another thing to keep in mind is the issue of depth of field.
If you do portraiture and fashion, you probably want the background to be out of focus, most of the time.
A wider aperture gives you this opportunity, while with an f/4 lens, you may have problems making it work. I say "may", because it's a matter of distance to your subject and the distance to the background.
This is where the fixed focal length lenses such as the 85mm f/1.8, 135mm f/2 and 200mm f/1.8 (sigh, sigh, sigh, give it back, Canon, give it back!) shine.
This isn't meant as a rebuttal to Jonathan at any level, it's just a few more things to consider in addition to what he wrote.