I' also a happy owner of both the 100L and the 24-105L, and I, too, was thinking "What the ..." when Canon announced the new 24-70 f4 L IS. Way too expensive for little gain.
lensrentals.com has an interesting comparison of 24-70 2.8 I, 24-70 2.8 II, Tamron 24-70 2.8 VC, 24-70 f4 IS, and 24-105 f4 IS, and that averaged over many samples. Here the new 24-70 f4 IS beats the 24-105 by a small margin in sharpness (and by a large margin in distortion
), and the Tamron lies in between the Canon 2.8s.
See for yourself:http://www.lensrentals.com/blog/2013/01/canon-24-70-f4-is-resolution-tests
Still, not only based on that numbers, I don't think that I would ever "upgrade" to the 24-70 f4. I would rather spent that money on the Tamron, which seems to be excellent, given the price.