Well, in that range I have mostly Nikon equivalents, but the experience translates reasonably well I think. With a 300mm f/4 prime lens, you have the choice of using it with a 1.4x TC unit to make a pretty good 420mm f/5.6 for added reach. You can also use the 300mm with a 12mm extension tube, or a Canon 500D + diopter lens (aka close-up filter--the current best of its type made) for added magnification in close-up shooting of things like butterflies, flowers, reptiles, etc. With the 100-400, your overall aperture range is from f/4.5 to f/5.6, so it's always going to bring with it slower shutter speeds than the 300 prime.
Alternatives would be the Sigma 80-400 OS, same aperture range as the Canon 100-400, and maybe the Sigma 100-300 f/4 EX HSM, which I also have,and which I think is a viable alternative to a p[rime 300/4. The 100-300 Sigma offers the focal length flexibility a zoom brings, with a constant f/4 maximum aperture, in a reasonably priced, normal weight lens; it is however, a tad softer at 300mm than the Nikon 300/4, BUT at a baseball game, having a 100-300 zoom range increases the shot opportunities many times over.
With longer zoom lenses, there comes a time when the maximum aperture becomes a critical factor, like late in the day, or under stadium lights; you will run into that critical factor with the f/4.5~f/5.6 lens much earlier in a baseball game around at twilight than you will with an f/4 lens. As cameras and their high ISO performance gets better and better, a lens that is only f/5.6 at its widest aperture begins to look much better as a potential sports lens than it does with a body that tops out at ISO 1600,so if you happen to have a latest and greatest body with uber high ISO capability, that too is a factor.
Sharpness and flare resistance wise-the Nikon 300/4 is a better lens than the 80-400 Sigma OS or the 80-400 VR-Nikkor or the Sigma 100-300 f/4 EX HSM,and so I am assuming that Canon's 300 f/4 will also have an edge over long zooms.