I've read the anatomical feature that allows humans a wider range of comfortable speeds with one base gait is bipedalism, and four-legged gaits have narrower speed ranges.

This is easy to observe for dog lovers. Walk with a medium size (say 50-60 lbs) dog off leash and you'll notice the dog has to choose between a gait that's a little slow for a human walk or a gait that'd be a fast walk for a human -- but you can still match either of those without much trouble. If you choose not to match your walk to the dog's speed, the dog will alternate between the two gaits to roughly match your speed.