In my experience..... trichomes reach their max potency about 3 weeks after they start forming. This has been the case with about a dozen strains i have grown.
However, the potency of the thc is not the only thing to consider. Terps, ripeness of the trichs have a distinct effect on the high.
Anyway... harvesting at 4 to 6 weeks wont cost you potency but it will have an effect on the type of high you get and the taste. But more importantly... harvesting that early means he is likely leaving a lot of weight on the hydroton table...