Hey, I use 1000HPS and I have the same issue about light penitration. These HPS create such a bushy plant that about half my plants struggle for light unless my plants are space 2-3feet apart.
You will not have a large deminisioning in returns JUST quality will be hinderd.. Some will argue this but most people have no idea since they cannot accually base there observation of there own to entrily correct. Every grow is not the same, little details stand in the way to mimic the exact same crop. Uncontrolled examples are the clones you get or take, eaither or, the next generation clones is older then the more vigerous one before..
Only trim the leaves that receive little to no light... Dont trim more then 20% of your plant at one time, and remeber the fan leaves are used to generate food inexchanging between roots and leaves. 4plants under 1000W is good, dont go less, but maybe go into a 5X5 or 6X6 to allow more plant speration... Otherwise keep doing what your doing, partly its the type of light your usisng that cause this sort of problem aswell... Long internode spacing when introduce into flowing.. HPS is great for curing any plant growth issue or odd internode spacing i found. good luck