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The 4-hi mod is for keeping the low beams on with the high beams. That's how the GMT900 trucks come stock, there is no 4-hi mod needed. (edit: at least that's how my truck was when I bought it)
As for the 6-hi mod, I'm actually pretty impressed with the output of the lights as they are compared to my NBS Yukon and Suburban so I never felt the need to pursue it. I did recently replace both of my low beam bulbs because one crapped out, and I was surprised to find a white powdery buildup on both of the bulbs that was really reducing output. Maybe check your bulbs for the same thing and replace them. I'm just running simple Sylvania H11s, not silverstars or any of the overpriced crap, and the increased output with the new bulbs was pretty noticeable.
I actually just looked at that, I was hoping to get on here before someone came on and made me look stupid. lol. 4Hi is factory. Now I really want to find out about the fogs on hi. Thanks.
lol sorry bro, wasn't my intention just sharing the knowledge.