If I am not wrong your car only has one coil right?? In other words every plug does not have its own coil pack?? Lets assume, one… If it keeps dyeing on you either you are getting cheap poor quality coils (you may want to spring for a new Toyota part), or the coil is overheating. I know on some cars if you dont put fresh dielectric grease on the bottom of the coil they over heat and die… Could that be the issue with yours? It could also be that there should be some kind of resistor on the power feed line that has burned out, sendint too much voltage to the coil and burning it out.
Coils are very basic, so for them to keep failing means you have another issue causing them to fail. So you are addressing the symptom and not the cause.