Sativa grow

Ive never done a sativa before, but since everyone says it takes longer to grow, I have a question. Im about 4 weeks into veg with my Urban Poison Strain (Nirvana), using DWC. I wanted to know, can I put this into flower now, or do I have to wait until the nodes start growing unevenly? What would happen if I forced them into flower now? Let me know, thanks!

12" is a good time to start flowering.

yea I know, but do I have to? Can I force flower earlier? Or will the sativa not flower regardless, unless it is ready? I know Im supposed to wait til the nodes grow unevenly, but if i do it earlier, will it still be okay?