4g or 4mg? Natural amount of secreted melatonin is much less (< 1mg), you're overdosing with 4mg. I use 0.3mg Herbatonin, which is bioidentical natural form of melatonin from a plant. 10-20mg is therapeutic dose.
I suspect some people just try whatever bottle they grab first, and may find either no effect, or unwanted effects (excess grogginess/nightmares), and never try another dose.