The 20/4 thing is true. I saw it in a scientific article. Any day length less than 20 hours will cause Cannabis to flower eventually. It just takes longer with, say, 19 hours. With 20/4 or longer they will stay vegetative indefinitely. Or you could use repeating 9/3 cycles. As long as the cycles coincide with a 24 hour cycle the plants will be fine. Like it couldn't be 9/4 or something. I use 6/2 myself. I don't like them to be in darkness for very long because plants elongate during the dark phases. The shorter the darkness, the less stretching. My plants are extremely short. No hermies or anything. They also go into flowering faster. They're actually getting longer light phases under 12/12 than they were in veg, so they adjust to it easier than going from very long light phases to much shorter ones as they would with 18/6 or 20/4 to 12/12. With my method, both the light phases and dark phases get longer when switched to flowering. This makes them flower quickly, without a 24-36 hour dark phase. I tried the 24 darkness and there was no difference.