The time of toppings (nodes) don't matter. Can top on the single leaf if you wanted. Ideally all you want is to create 2 Cola's (tops).
If you are waiting for the 5th node should be pinning down the top so side branches grow out more.
From personal experience, I would wait until week 4 before topping. You want 5 nice and long nodes to work with. Since it's a photoperiod plant, you have time to veg it properly for regrowth.
Hi!
So you are going to top the gals?
Then yes, cut the plant in half above the fifth node, it does not matter if you leave some extra stem in there and you wont have to cut "just" above the brnaches