The EF 300mm f/4 L IS USM is a nice lens. You already have a lens that covers that length, but the 300 has a nice minimal focus distance (for butterflies and stuff), is lightweight and quite fast I think. The IS on it is REALLY nice and when you put a 1.4x converter on it, the images still look real nice! Not sure if that's the one to go with (the 100mm L I think would be great for you) but it's a fun one and wanted to mention it! It also has a nice built-in lens hood. You could go with that or the 400 f/5.6 and get rid of the 100-400... though that might be a bit dangerous. The 100-400 is a real nice wildlife lens.