I met 2 situations in 4 player FFA (3 piece new nat+10 disc cloak+SoJ+10 disc quiver sometimes):
-I outgear them, so I stay in middle and only use smoke screen sometimes, maybe even use sentry to own them more
-They have similar/better gear, so I let them fight each other and snipe them from far away, legacy nats give different discipline management ability, you can vault to get in better position but your dps/ehp is obviously lower, you can get away for 10sec and get your discipline back. With new nats, you can't really vault and smoke screen, you have to manage your discipline better.
I think legacy nat might be better in 4 DH FFA and new nat is better against good ehp barbs/monks and other classes.
I played against 3 other paragon 90+ DHs and it was the most insane 1-shot fest I've ever seen.

And the only counter to a barb is high dps, you have to burst them and not let them run away and regen their life.