http://www.the-digital-picture.com/Reviews/Canon-EF-24-70mm-f-4-L-IS-USM-Lens-Review.aspx
At 24mm with a wide open aperture, the 24-70 f/4L IS bests the 24-105 L IS in sharpness by a modest amount. The 24-105 L delivers a sharper image at 50mm f/4. Sharpness improvement at 70mm brings the 24-70 f/4L IS back up to near equality with the 24-105 L. These two lenses perform more similarly at f/5.6 and at f/8 where their results are nearly comparable. Again, the results are similar at f/11.
The 24-70 f/4L IS shows less distortion than the 24-105 f/4L IS - and significantly less at 24mm. Landscape photographers shooting over a flat horizon (such as water) will especially appreciate the difference at their most-frequently-used focal length: 24mm. Having a distortion-free lens makes leveling the framed scene much easier and reduces the destructive distortion correction needed during post processing.
There will be a small amount of CA (Chromatic Aberration) in the peripheral image circle at 24mm, but the 24-70 f/4L IS is nearly void of CA until the longer end of the focal length range where mild CA becomes apparent at 70mm. The 24-70 f/4L IS has noticeably less CA than the 24-105 f/4L IS at the wide end, but more at 70mm.
The 24-70 f/4L IS has less vignetting than the 24-105 f/4L IS at the wide end at f/4, but more at the long end. By f/5.6, the two lenses are close in this regard. The 24-70 f/2.8L II, with its 1-stop wider aperture, has the comparable-aperture vignetting advantage until about f/8 where the two are nearly equal.
This lens features Super Spectra coatings to reduce flare and increase contrast. While the increased contrast part appears successful, I'm not as amazed with the flare performance from this lens. You will want to avoid the sun in the frame at all focal lengths and apertures - unless you want the artistic effects of flare. The 24-105 L and 24-70 L II show slightly less flare, but not enough less to be a differentiating factor.