What is the difference between the EF 70-200mm f/2.8L IS II USM and the EF 70-200mm f/4L USM? The only difference I can see is that one is longer and cheaper?
The first has a f/2.8 maximum aperture and image stabilization, while the second has a smaller f/4 aperture and no image stabilization. The larger aperture needs larger lens elements, and a good image stabilization system also adds cost. Between the larger aperture and IS, you'll be able to shoot handheld in lower light with the pricier lens, and the larger aperture also gives you shallower depth of field for better separation of your subject from the background. The f/2.8 version also has better weather sealing.
Canon also makes a 70-200mm f/4L with image stabilization and a 70-200mm f/2.8L without image stabilization, both priced around $1200, so if you want the larger aperture without IS or don't mind the smaller aperture but want IS, you have those options.
And why is the EF 70-300mm f/4-5.6L IS USM cheaper than the EF 70-200mm f/2.8L IS II USM? I would have thought that the EF 70-300mm f/4-5.6L IS USM would have been more expensive.
The larger f/2.8 aperture on the 70-200mm f/2.8L IS II USM requires significantly larger lens elements. As well, notice that it's f/2.8 throughout its range, whereas the 70-300mm f/4-5.6 IS USM is only f/4 at the wide end of its range, and goes down to f/5.6 at the long end.
Also the same thing with the EF 200mm f/2.8L II USM and the EF 70-200mm f/4L USM? The EF 70-200mm f/4L USM looks a lot better and is cheaper.
The 200mm f/2.8 is a prime lens rather than a zoom, so it's going to be sharper. With it's larger aperture, it's one stop faster. The trade-off is that it's not a zoom, so you lose the convenience of being able to zoom out to a wider field of view.
The 70-200mm f/4L is IMO a great lens, especially if you're on a budget and can get along without IS. There's one in my bag right now. It's smaller, lighter, and less expensive than it's f/2.8 siblings, and if you're shooting moving subjects in daylight (say, kids playing soccer) you're probably using a shutter speed that doesn't require image stabilization anyway. That covers a lot of what people do with a telephoto zoom, and depending on how you look at it, it's either 1) easy on the wallet, or 2) a gateway drug to the world of Canon's L lenses. But the other lenses you mentioned have important advantages and are worth a good look if those features are important to you.