Can you add fitness 4-7 days out from a 100m-400m race? Or are any workouts done for maintenance only?

CNS continues to need a stimulus or you will lose sharpness. That's the purpose of those last few workouts. Without them, you really will lose some speed.

It's true that you aren't really going to gain any fitness in the few days before a race. You need to keep what you already have and any 100% sprint work would have a negative impact on your race. Practicing your block starts and running out to 20 and 40 meters should definitely help. Do a search on You-Tube for the best 100 meter block start training. I have seen videos of an excellent coach who talks about not trying to run upright too soon and to stay in a lower position and spring forward like a bullet. When you stay lower, pushing forward off of the track you are more aerodynamic and get more power than if you try to start running up right too soon. Remember than in any kind of running your power comes from your foot pushing off the ground to drive yourself forward. You are not trying to reach forward with your stride. Your feet are under you driving you forward. Good luck.

Charlie Francis discussed this in his discussion of the ten day taper. Also, I discuss it in detail in the Sprinter's Compendium. If you want to know more I would check that out or the website simplifaster