Pros generally favour zooms with a maximum aperture of f/2.8. As a result, manufacturers like Canon deliver up plenty of f/2.8 glass designed for professional photographers. But Canon has a long history of producing f/4 maximum aperture zooms for pros too, dating back to the days when FD manual focus lenses were the company's mainstay. Up until now, the best example of this has been the EF 70-200mm f/4L USM, which offers excellent optical quality in a lens that is considerably smaller, lighter and less expensive than Canon's two 70-200mm f/2.8 zooms.
Canon's latest f/4 pro zoom is the EF 17-40mm f/4L USM. Assuming the optical quality is equivalent to the EF 16-35mm f/2.8L USM - and the use of Super UD glass suggests it could be - then the new 17-40mm would appear to be a less expensive alternative to the 16-35mm, though with a slight focal length range difference. Its size and weight, however, don't seem to benefit much from the f/4 maximum aperture, so the EF 17-40mm f/4L USM may not be a good choice for Canon shooters seeking a trimmer alternative to the EF 16-35mm f/2.8L USM.