I think the more plants you have squeezed in per sq ft, the more you have to trim. For 4 plants per sq ft SoG, I think the rule of thumb (or should I say the rule of Al B Fuct) is that all growth on the lower 1/3 of the plant, plus any growth shoots longer than 1". This will give you a main cola, a few top colas, and some buds along the stem. What you want to avoid (again, assuming you have lots of plants packed pretty tight) is any branches that will form budsites of their own.
