No, you have it backwards.
Shorter Flowering Period = Greater THC:Other Cannibinoid ratio. More THC and less other chemicals leads to a more "sativa" or "heady" high.
At the same time, don't think that if you pick 4 weeks earlier than the plant's supposed to go (all times are averages, there is nothing set in stone), that won't increase its "up" potency...
Check trichomes. When they're developed with little bulb-like tops (capitate stalked trichomes) and translucent, they're more "up"...the more ambered and dark they become as they mature, the more of an "indica-like-down" it'll become.
Good luck to ya!