I would correct that even further.
It isn't about the fittest or death risk, it's about being able to procreate and survive.
In your species example of the 4, 6, and 10 mph creature. If the live birth rate of the creature declines as their speed increases (musculature takes energy/hormones away from breeding, high speed movements cause more lost pregnancies, etc...) than the 4mph species may actually be the winner as they will out-bread the 6 and 10mph variants.
Now, throw a 5 mph predator into the mix and the picture may change. If the 4mph variant can still breed fast enough to offset the deaths to the predator and out populate the higher speed variants, then it could still be the winner.
More likely though, the 6mph critter would win out as it is able to out breed the 10mph critter and would suffer significantly less losses than the 4mph critter to 5mph predators.
It all comes down to procreation. Which is the basic of the movie Idiocracy.