I wish it was f/2.8...
Just out of curiousity, why? I mean, practically, and not just because you want a collection of f/2.8 or faster lenses
24mm is too wide for events & people (where you might want the speed) due to the distortion, so odds are 99% of the use of this will be for landscape. Landscape is f/8+ most of the time... And the wide side we already have the 16-35mm f/2.8 for events & people - whose focal length is actually long enough to shoot people without unflattering distortion.
It is possible the f/4 design might reduce cost, size, weight, and actually improve image quality at smaller apertures where this would be used 99% of the time.
I've used the Nikon 14-24mm for events and people. I hoping was that Canon make a zoom similiar to that..yeah yeah the 16-35 good glass but the Nikon outshines it. The 16-35 mm I've tried it and "meh". I agree it would be used mostly for landscape. I have a streak of creativity.... I'm buying it regardless anyhow if this glass become reality.