If you activate Dark Hunter's 5 or more effect and an effect is chained that reduces the number of DARK monsters in your GY to 4 or less, Dark Hunter's effect still resolves.
If you activate the '5 or more' effect of "Dark Hunter" and an effect is chained that reduces the number of DARK monsters in your GY to 4 or less, the effect of "Dark Hunter" still resolves.
While "Skill Drain" is active, all of the effects of "Dark Hunter" are negated because the primary effect, 'This card's effect changes depending on the number of DARK monsters in your GY', is negated.