Honestly, flex has very little to do with where you ski. Finding the correct flex has more to do with how much you weigh, your ankle flexibility, and what younare used to.
For example, 2 skiers, same exact skill level, skiing the same exact terrain but one weighs 120 lbs and the other is 220 lbs. Would they use the same tongue set up for park? Or charging big lines? No way. A 4 might be perfect for park for the lighter guy but way way too soft for the bigger guy.
My point is, it doesn't make much sense to ask what tongue should you use for park and what tongue should you use for powder. It would be more of "what tongue should I use if I am 150 lbs and used to skiing in 100 flex boot"? Once you fin the right flex tongue for your subjective needs it will be great on any terrain.