how to trim

I am currently on my 12th day of flowering. I have 4 Northern Lights in a 4xt tent and it is filling up quick. Can someone give me some advice in trimming? This is my first grow and I am worried that my light isn't able to penetrate enough due to all the leaves, is this the case? I am running a 600 watt HPS light. I Don't want my yield to suffer and I also don't want to go an just start chopping away so any and all help is greatly appreciated. These pictures were taken a couple days ago and since then they have filled in even more.
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You could pull the pots further away from each other that'd help and any fan leaves in the way you could bend them out the way or just nip them off the odd 1 or 2 here n there isn't going to affect anything.

I have the pots spread out and the leaves are pretty much touching the sides. Should I trim up the bottom of the plants so they are not wasting their energy and nuts on the lower leaves? I belie this is called lolly-popping? I just don't want to hurt my plants.

the lower older/smaller growth that isn't getting light can be taken off to help with air circulation, keep the tools clean.

Your canopy isn't that thick.. remove all of the lower growth then bend the fan leaves out of the way ... I keep a thin canopy because of high humidity... just remove enough to keep good air flow and keep the leaves from laying on top of each other...