Well, those 2 lenses are targeted at different markets.
The 24-70 f/4 is targeted at the lower end of the market where IS (shooting at slow shutter speeds or have shaky hands) and wants closeup/short 'macro' operation with fairly good image quality, but not top end.
The 24-70 f/2.8 v2 is targeted at the high end professional that needs to be able to gather as much light as possible to keep shutter speeds up, build extremely tough, and top end image quality (especially sharpness, this lens is super sharp wide open across pretty much the whole zoom range). And it has a price commensurate with it's qualities.
Thank you for that really good break-down. Do you have either of them or what's your preference?
I've already got the 24-105 f/4L so I don't see a need for the 24-70 f/4L IS for me, although from what I understand it's a bit sharper and does have that close-up mode, although I'd lose 35mm worth of reach.
I'd really love to get the 24-70 f/2.8L v2, and eventually I will, but it's rather expensive for me and I see more of a need for the 70-200 f/2.8L IS v2 first for me. And perhaps the new Tamron 150-600mm...so many lenses, so little money...