I am not going to waste my breath arguing the plausibility of fast zombies, I am just going to attempt to formulate a plan, or possibly a guide to surviving a fast zombie invasion. I am going to go out on a limb here and make a few assumptions about fast zombies.

The fast zombie is created by a mutation in the Solanum virus, thus it would not be encountered until the later stages of a class 4 outbreak if at all.
There are a few key differences in the behavior and anatomy of a fast zombie versus a slow zombie

1. Unlike the slow zombie, the fast zombie HAS to eat.

2. Because it must have a working metabolism to sustain these levels of activity the fast zombie must also have some functioning organs.

3. If a food source is not readily available a fast zombie will go into a dormant state where it is indistinguishable from a slow zombie. If it remains in this state to long without consuming biomass it will remain a slow zombie permanently.

4. They have a limited climbing ability i.e stairs, low barricades, steeper grades but no trees ladders or high walls also they can jump but the probability of them successfully jumping and grabbing a hand hold is unlikely.

5. They may resort to cannibalism (eating other zombies)in order to maintain their increased activity level.

6. A fast zombie is created at the point of infection. Because a fast zombie needs functioning organs, a slow zombie cannot become a fast zombie, a surviving human must be infected with the mutated virus and have intact organs, if these requirements are not met, then the infected human will become a normal slow zombie.

7. Though they have improved motor function, they cannot swim, going under water will cause their lungs to cease functioning, which will permanently make them a slow zombie.

8. They possess very limited cell regeneration which means that they decay at a slower rate then their slow counterparts, but they are still decaying.

9. They best way to kill a fast zombie is still by destroying the brain

10. The ability of a fast zombie to run is, as always, determined by the level of fitness of the infected human. Having said that, the top speed of the zombie will decrease over time depending on the level of wear.

If you are in an exposed position and you encounter a fast zombie, either shoot it or flee to a safer position if one is readily available, engaging a fast zombie in close combat is not advised unless you are highly skilled in melee combat or you have no other options. Even if you can run faster than the zombie, it can run longer than you can so your best option is to evade or destroy it. If you can't outrun it and can't fight it try rapid changes in direction. It's motor skills, while an improvement on the slow zombies, are still inferior to yours. If you get behind it, it will take several seconds to register your the fact that you're behind it, stop, turn around and get back up to speed, just make sure to stay out of reach.

Strategy for class 4 outbreak with confirmed occurrence of fast zombies at +25%

The only way to survive any class 4 outbreak is to have already formed a plan and begin putting it into action when the outbreak is at class 1 or 2. If fast zombies are involved, then the only way for you to raise your chances of survival above negligible is to find a location that is intrinsically safe. That means that it has to be either inaccessible by land such as an island or boat, on terrain too difficult for most zombies to traverse such as on a mountain, too inhospitable for zombies to survive for very long such as the arctic circle, or too remote and expansive for it to be worthwhile for many zombies to bother traversing it such as a desert or tundra. The point that I am trying to make here is that your urban fortress just wont cut it, at least not for very long and really, what kind of life is that anyway? aside from a short one I mean. Just sitting in a hole waiting to die, really your only hope is if somebody more resourceful than you decides to come to your rescue. Sermon over, I'm just saying that if you live in an urban, or suburban area, formulate your plan around escaping to an area that is easier to defend.

Okay, so you have made your escape and have arrived at whatever location you think you can best defend and you're wondering what to do next. Now is the time to make your stronghold. If you are on land then the first thing you are going to want to do is find or build a building that will keep you and your group relatively safe. If you have chosen your location wisely then it is unlikely that you will be facing large groups of zombies. At least not initially The one thing you are going to really want when establishing your stronghold is visibility If you have trees obstructing your view, cut some of them down, besides, the lumber will come in handy.

The next step hinges on finding other survivors. Sure, you might be able to have isolated pockets of humanity that persist for a few generations, but eventually humanity would die out. Therefore you must find away to seek out and at the very least, communicate with other survivors. The only way for humanity to survive is to make a coordinated effort to take the fight to the zombies.

This point is where things get a little different. The best tactic for taking out slow zombies is to bait them into going to a location with a bottleneck and shooting them at a distance, this tactic will work with fast zombies, but a contingency will have to be in place in the event that a large group of fast zombies get too close. One example would be bringing down whatever you are using as a bottleneck down on top of them (buildings, cliffs etc.)
Of course, depending on how long it took to start a coordinated human effort, there might not be any fast zombies due to scarcity of food, but assuming there are it is best to take the extra precautions.

As far as urban cleanup goes, fast zombies complicate things, but there really isn't a need for major changes in tactics other then taking extra precautions. Always remember to knock before entering a room and always protect your escape route. Fast zombies pose the most danger post cleanup and re-population. If you have dormant fast zombies carrying the mutated virus wandering around, then anyone bitten by said zombie will become a fast zombie, and the rate of infection will increase exponentially unless it is stopped early on. A second outbreak consisting entirely of fast zombies is virtually a no win situation if it got out of hand. The only thing remotely approaching victory would be to let the outbreak run it's course, which may take decades and start over from scratch.

Okay, I wrote this kind of spur the moment and I know that there are some serious issues because I just sort of made a lot of it up as I went along. I think the only part of this that is remotely useful at this point is the very beginning where I outline the differences between slow and fast zombies and my little warning about a second outbreak at the end. The middle is pretty much just filler until I think of something better and please, critique so that I can improve upon it.