Ef 500mm f/4L IS usm or Ef 200-400m f/4L IS usm Extender 1.4X

If only one for wildlife?

I presume you are referring to the Mk II version or Mk III of the EF 500mm f/4.

I would personally always go for the 500. In my experience you are always wanting that extra reach for wildlife.
So from my point of view it is comparing when using the 1.4x TC, 560mm to 700mm. With 25% more focal length you are getting 56% more pixels on your subject. Also at 700mm you are still at only f/5.6.

I have owned the 500mk2 version and it a wonderful lens and a joy to use, but I still needed that extra reach. So for the past 2 years I have been using the 600mm f/4 mk2 version. The zoom is an advantage over the Prime but for what I photograph in the UK, I've never found myself wanting less mm.

The 500 f/4 is also lighter and will be faster at focussing than the 200-400.

As above, plus...

With the 500 you can get to 1000mm/f8 so can still AF, the zoom maxes out at 800mm for f8 autofocus.

And the 500 is a much better balanced lens, I found the zoom very front end heavy in comparison, and a lot harder to swing around to follow fast moving subjects when I fad get change to try them back to back.

500mm

Save some money, save some weight, and gain some reach.

That said, when I do Africa safari, I have the 500mm on one body, and the ever so point-able and flexible 100-400mm on another most of he time.

Clarence54 wrote in post #18918417
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Ef 500mm f/4L IS usm or Ef 200-400m f/4L IS usm Extender 1.4X
If only one for wildlife?
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This decision really depends on what kind of wildlife you are going to photograph, and what conditions you are going to be photographing it in.

For my own wildlife photography, the 200-400mm would often be a better choice for Whitetail Deer, Bighorn Sheep, ducks in flight, various species of Grouse, Mountain Goats, Lizards, toads, frogs, turtles, Elk, and Ground Squirrels.

The 500mm, both with or without a 1.4x or 2x extender, would often be a better choice for Mule Deer, ducks on the water, songbirds at feeder setups, tree squirrels, cavity-nesting birds, "walkabout birding", Pika, shorebirds, Coyotes, raptors, Beavers, and rabbits.

I strongly disagree with comments like, "you always need more focal length", because there are so many times when you DON'T need more focal length and wish you could shoot wider because you end up so close to the critter or bird that you can't fit it in the frame.

I have a 100-400mm and also a 300-800mm, and for wildlife I use the 100-400mm more often because it covers a focal length range that is more useful for most of the wildlife photography that I do. . I am often as close as I want to be and having more "reach" would not be helpful at all. . In fact, I usually use my 100-400mm zoom at less than 400mm. . But of course there are times when I need the longer lens because 400mm sometimes isn't long enough and in those cases having more reach is absolutely necessary.

In your post you say, "if only one for wildlife", so I assume that you do not want to pay for two "big money" lenses. . If that is the case, then my suggestion would be to buy the 500mm prime and then to also buy the much cheaper 100-400mm v2 to accompany the big 500mm. . If you can afford to buy a $10,000 lens, then scraping up another $1500 for a used 100-400mm v2 shouldn't be much of a problem. . A 500mm and a 100-400mm would be a great combination.

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I would go for the 200-400 with the inbuilt converter one. This zoom range helps to compose the shots properly and the optics quality is really good.