What's going to happen if you're over 100% total power and you set the KWS to priority 4 is you won't be able to use it.
This is what priority does; When over 100% you need to set a priority to certain systems so that when you unsling your weapons your circuits don't fry and turn off your whole ship. To achieve this you need 2 priorities, because there are only 2 states for the ship. Weapons in or out. Note when I say weapons this includes the KWS, wake scanner, cargo scanner and discovery scanner. If any of these modules, along with any weapons, are set to priority 2 then they will deactivate once the hardpoints are deployed.
Then we have damage control priority management. This is extremely useful for cold running combat pilots, because damage to the power plant can affect it's power output, but can be used for shielded ships just the same although the chances of power failure are of course much lower. This is where the multiple layers of priority really count. As the power plant takes damage output can vary, so if your ship loses just a bit of power you can be set to lose say your lifesupport at 5, weapons 4, power distributor 3, sensors 2 and thrusters at 1. This way you can keep the ship running as long as possible while taking damage to the power plant.
Hope this helps.