The 2pc BM bonus is that your dire beasts get the bonus from Bestial Wrath as well; it's the 4pc that gives you the additional reduction on BW's CD when you pop DB (side note: feels real weird sometimes to see THAT many orange-outlined pets in melee range, even if "THAT many" usually ends up being three instead of just two, lol)
That said, I've been leaning toward using 2pc/4pc over higher ilvl items in those slots, but I also don't have the pants so it's been less of an issue for me. I'm not sure if that works out in every case. AskMrRobot
has a "Best in Bags" option that I've learned to mostly trust (once I get far enough away from the scaling HFC heirloom trinkets and the Perfect Dreamgrove Blossom, and they've had time to get stuff in their database), but it might be a premium-only feature; if you've used the free week trial they offer already, you could always do something weird like wearing one piece of tier and seeing how much "seemingly-arbitrary AMR points" putting on a second piece gives you and if it's a largeish number then that may be the way to go.
(Alternately, you could also just get someone who does have premium AMR to run your character through, lol :P)
[ Basanti - Draconic Savant, A-Korgath(US) ]