Is it just me, or does this seem like a lens for a body that does not yet exist: a budget dual-pixel AF full-frame camera? Otherwise, given that the f/4L is available for such a good price, has ring USM, has constant max aperture and has "red ring" cachet/build quality/sealing, I don't see the point of this new lens. Maybe it's intended for 70D or 7D II use, but it does not seem like a very useful focal length range for crop sensors.
Or, they could be planning to update the 24-105L and price it higher - say $1400.
In that case it makes sense to have cheaper alternatives - like the 24-70/4L and the 24-105/STM.