Fdd had a great thread so when you're serious you should read that one but briefly I think it's best to top as soon as the branches stop cominig out in pairs and start coming out in alternating pattern, ie one on one side small space, one on the other side. topping for four is what I recomend having had great success with it. remove any weak lower branches. The first two are usually pretty sickly looking comapared to the rest. then leave the next four branches that you have chosen and cut everything off from that point on. Just go ahead and cut the top off. You can use the top as a clone. That's it. Real simples. you will now have a plant that will produce four strong colas in a nice tight pattern. Promise!