1.) Get the 24-105 f/4 at the nice discount for package/kit deal, or
2.) Just get a prime (85mm?) (50mm?) and live with it, or
3.) Wait for 24-70 f/4?
You should look at your current shots and determine how often you go from 70 to 105, i.e. if loosing the zoom range is an inconvenience even if you've got a 70-xxx lens.
You didn't write about your budget - Canon's 24-70/4 as a new lens is overpriced but seems good because of the newest IS system, near-macro capability and and supposedly better iq as the "old" 24-105, but the latter is an absolute steal when in a discounted kit (there are also lots of used lenses around). Otherwise if you want f2.8 for whatever reason on a budget have a look at the Tamron.
He's already using a 24-70 (effectively), and a 70-200mm. Losing the long end (losing the crop) on the 70-200mm may be more of a concern. He may want to consider a teleconverter even?
The OP may even want to consider buying the kit, and then reselling the 24-105mm that comes with it. It's what I did, and I sold it for $850. That'd get him most of the way to the 24-70/4 IS or the Tamron 24-70/2.8 IS.
If he is coming from a 17-55/2.8, despite the ISO advantages of the 5D3/6D, there's a good chance he may still want IS and 2.8. If he is shooting evening landscapes or evening family group shots hand held, then any aperture/IS advantage will likely come in handy. Going from the Tamron 2.8 to the Canon 4, would be the difference between 3200-6400, or 6400-12800, which is certainly noticeable if printing larger. Noise reduction would wipeout any sharpness advantage of the 24-70/4, doubly so the 24-70/2.8 as it doesn't have IS.
He may want to trade sharpness for the aperture (and thus noise) advantage, depending on his shooting habits.