How do I unshield an ion cannon in a map?

[zubin]

I have a shielded ion cannon in a map I'm trying to create. I want it to become unshielded after the player destroys 4 separate generators. How do I set this up in the editor exactly? I want to have it so it doesn't matter in what order the 4 generators are destroyed (just as long as all 4 are destroyed) but am unsure how to set up triggers to do this or should I try to do it using some other method?