Me personally, I only bought my 16-35 in the past 3 weeks. I was keen to see how the f4 lens tested, and it seems to test well.
The way I see it, if you are shooting landscapes/architecture, you would be shooting at f8-f22. And within that range, the differences between the lenses are negligible. In particular at f11, there is only minute differences in corner sharpness.
Both lenses are great stopped down, the f4 will not do f2.8 though, and I have been burned in the past with f4 lenses to be honest - on paid work too I might add.
So for me, it is f2.8 or nothing from now on. If I am shooting landscape/architecture, I stop down to f11 and get 98% of what the f4 lens would get me. I would never use f4 as a wide aperture as it just isn't wide enough, so I am keeping my f2.8 lens for the times I need a) the shutter speed or b) the subject isolation. IS cannot do either of those things on an f4 lens.