After using the f/2.8 for a couple of weeks, I decided to the sell the f/4. The IS is noticeably better and for me, having f/2.8 as an option makes it much more versatile for me. I would typically bring the f/4 to my event and wildlife work, but would also have to bring my faster prime lenses, whereas now I can just bring one lens. So the smaller lens actually ended up taking up more room and being heavier because of the need to bring my 135 and other lenses with it.
If I was just interested in landscape or travel photography, the f/4 (along with the 17-40 and 240-105) would do me fine, but for the work I do, the f/2.8 seems to be the lens I've been missing!