I have a 70-200mm f/4L IS zoom along with two long primes: 300mm f/4L IS and 400mm f/5.6L. I love them all but, if I had only one lens for wildlife, I would opt for the 100-400mm f/4.5-5.6L IS because of its versatility.
However since I shoot with two or more cameras regularly, my three lenses work for me.
One plus for the 300mm f/4L IS and 400mm f/5.6L primes is that they have built-in sliding lens hoods. These are a joy to work with in comparison to the separate bayonet type hood. They are especially nice when using a CPL filter because I can slide the hood back, rotate the CPL and extend the hood all in a split second.
I wish my 70-200mm f/4L IS has that type of hood.
BTW: As per Amazon prices in U.S. Dollars: The 100-400mm f/4.5-5.6L IS = $1639, 400mm f/5.6L = $1299, 300mm f/4L IS = $1,359 and the new 70-300mm f/4-5.6L IS = $1,499. The last two lenses can be a bit chort for a lot of wildlife, especially birds but the 300mm f/4L IS accepts a 1.4x TC without great loss in image quality and auto focus speed and accuracy. Of course, the TC costs extra. OTOH; the 300mm f/4L IS has a very close MFD and is a dandy close up lens, especially when fitted with the TC, an extension tube or a 500C close-up filter.