Top above the 4th node, auto flowers or photoperiods. I did five auto grows and the conventional wisdom for timing for autos was 21 days. That worked out pretty well for me.
No worries about "slowing down" growth. Topping removes nothing vital from the plant (obviously). For the life of me, I can't figure out how removing foliage that has done nothing to contribute to the growth of the plant can "slow growth down".
Top at four nodes and the other foliage will continue to do what that growth has been doing for the entire life of the plant - powering growth.
UPDATE: So I've been rotating the pots and plants as a form of very low stress training to encourage a uniform circular growth shape of the plant..I also started EVER SO GENTLY bending the top 2 leaves away from the center top to allow more light...anyway just thought I'd document it....
As a "high light" grower (just made that up), I applaud the desire to get the best light you can on your plants but there is no need to bend things to let more light in at this stage. The best thing you can do, in terms of light, is make sure that you give your plants plenty of it.
The only way to determine how much light your plants are getting with is you measure it. Vendor
guidelines re. hang height and dimmer settings are good
guidelines but the only way to know how much light your plants are getting is to measure it.
You can drop $300+ on a PAR meter (there are cheaper models available and there are reasons why they're priced as they are) or you can get a light meter for about $35. A PAR meter is an excellent piece of equipment but, for most growers, a light meter is the better purchase and is, in practical terms, as accurate as a PAR meter.
When I started growing (again) two years ago, I found this graphic very helpful. I used it as a "how to guide" to refine my growing processes. It's a screenshot of a YouTube video by Dr. Bruce Bugbee. He is, as best I can tell, the leading expert on cannabis research. His "intro to grow lighting" videos on YouTube are superb.
