You should top your plant anytime between when the 3rd & 5th set of leaves develop. Every plant strain reacts differently to topping, so when first starting off with a new strain, or if you have never topped a plant before, experiment. Try topping at different times, and different quantities.
When FIM'g I found its better to err on the side of topping, cause if you don't take enough, nothing happens, the veg stretch gets slowed down, thats it. Single main stem comes back. . (if the plant has to be traumatized, might as well get something out of it)IMHO
Selecting the point for topping to get 4 main colas.
To get 4 main colas, let your seedling or cutting (clone) grow to about 5-6 nodes and pinch out (cut) the stem just above the 2nd true node. The node where the cotyledons attach doesn't count. The result will be a redistribution of the auxins and other hormones that normally collect in the tissue of the terminal leader's tip. These hormones will be redistributed to dormant buds that reside in the nodal axis where the leaf petiole attaches to the "trunk", below the cut. The new foliar output response will be quick (within 24 hrs., see photo below) if you have a healthy growing seedling and will be your future main colas - 4 instead of the usual 1.
Here's a photo of the same plant about 8 weeks into flowering. The plant is about 42" tall, has 4 main colas with an abundance of large, healthy fan leaves.
Notice how the weight of the heavy colas is naturally pulling them apart, which opens the plant up so that light can penetrate the interior of the canopy. This is the same principle used by fruit orchard managers who create an open vase profile for their trees in order to increase production. This profile also has a side benefit of providing good interior air movement which reduces fungal/rot pressures.
(by uncle ben).