In case anyone stumbles over this old thread at some point and is interested to know how the story ended ...
Having done some more testing, I'm pretty happy with my 24-70 4 IS now. The IQ is pretty consistent all the way through the zoom range now. It's possible its weak spot is now at 35mm (f/4), and I think it might not be quite as good at 24mm (f/4) as before I sent it to Canon (but it's marginal - I might be imagining that), but really its consistent - and very good - throughout. It's a vast improvement over its performance before I sent it back to Canon, that's for sure.
It's interesting reading around the internet that some people report the 24-70 4 IS being great throughout the zoom range (eg see PhotoNet http://photo.net/equipment/canon-ef24-70f4/
, and see SLRlounge's "lens wars" series including their 50mm "wars"), while many others report it being weak in the middle of the zoom range, especially at 50mm. Sporgon mentioned in an earlier post that Roger Cicala had found the 24-70 4 IS has an unusually high number of adjustable elements. Maybe Canon bit off a little bit more than they can chew with the 24-70 4 IS, in the sense the QC required to make it consistently good out of the box would mean it would have to be priced at a level which isn't going to fly from a commercial point of view?
Anyway, the story has ended well for me, so I'm happy!
PS - It seems pretty clear it's not going to be what you're looking for if you're serious about macro photography, but as someone who isn't that much into macro, I'm definitely having some fun with the (semi) macro mode on the 24-70 4 IS.