Alistair Windsor , Jun 19, 2006; 03:58 a.m.
Yes. The f4 is the smallest f number (largest aperture).
In general f2.8 is "better" than the f4 though it all depends on the lens. The f2.8 lenses tend to be heavier and bulkier than their f4 counterparts.
The 24-105/4L IS is the only full frame standard L zoom with IS. I would argue that its 2.8 counterpart, the 24-70/2.8, is slightly better. Slightly faster for low light, shallow DOF for portraiture, and almost no vignetting at the wide end. It lacks IS and a little reach.