Proper Prunning methods?

Hello, i will start by telling how happy i am i found this site it has helped me emensly !

I have an indoor grow room with 4- 120w grow lights, i have 6 plants that have been vegetating for almost 7 weeks, ranging in size from 1 ft- 20 inches, my goal is to have my plants no bigger 3 1/2- 4 feet,, A. how, or should i top them? does prunning help growth?
 
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