The f/4 lenses are good lenses and represent a good value. There might be situations where you can tell the difference, but it depends on subject and style.
The ultra wide choice between the 14-24 f/2.8 and 16-35 f/4 VR is a good example. The 14-24 image quality is superb - even in corners. The lens does a great job controlling aberration and flare. But the 16-35 has filter threads supporting easier use of Grad filters circular polarizers, and ND filters. It also is much lighter, less expensive, and VR allows better use handheld with lower light levels. I chose the 16-35 since I do a lot of stream photography in the Smokies. For architectural work, the 14-24 or a tilt shift lens would make sense.
The 24-70 is a solid choice and tough to beat. While the 24-120 has good image quality, I find the 24-70 is my bread and butter lens and place a lot of value on both f/2.8 and the image quality.
The 70-200 f/2.8 is very sharp at f/3.2 to f/3.5. It's an excellent lens - my favorite. The new 70-200 f/4 is a very good alternative if you don't need f/2.8. The new 70-200 f/4 does not ship with a tripod foot, so it is a relatively expensive alternative.
I'm a little uncomfortable suggesting where you need to draw the line on quality, cropping and other tradeoffs. In relative terms, the differences are very small. My guess is other factors - weight, cost, filters, etc. - may end up being as important or more so than image quality.