The biggest problem the 24-70 f/4 has is competition. The 24-105 suffers from the same problem. Canon has to heavily discount the 24-105 in kits to move them. That is where the 24-70 f/4 is going.
Want faster zoom? 24-70 VC, 24-70 2.8L, 24-70 2.8L II, 28-75.
Want faster? Too many options to list.
Want better IQ: 24-70 VC, 24-70L II. and a bucket-load of primes.
Want more range: 24-105 x2 (Canon and Sigma), 28-135.
Want to spend less money? 28-75, 28-135, Σ24-105.
Want to do video? The 24-105 STM is due in two weeks.
The EF 24-70 f/2.8L II and the Tamron 24-70 f/2.8 VC are unique in their class. Therefore, they command a premium price. The 24-70 f/4 IS is a mediocre lens in a sea of mediocrity. I can put this lens to shame with two cheap primes: the 35 2 and the 85 1.8. Personally, I chose to spend a bit more money to get the 35 IS and the 100 f/2. Both of which give me two stops more light, to use however I want. I have a Tamron 28-75, which at f/4 is at least the equal of the 24-70 f/4 in the center 2/3 of the frame. It also does f/2.8 (Ok, not really well) and cost me less than $300. If I need a bit more focal length, I have a 28-135 or a 70-200 2.8 OS to choose from. One is 1/4 the price of the 24-70 f/4 and has better range. The other, again, is faster, and at its best where the 24-70 f/4 is weakest. Expensive mid-range zooms like the 24-70 f/4 offer shooters like me absolutely nothing. When the 24-105 STM is released, the pool of potential buyers gets even smaller. Apparently, some folks do video with SLRs.