2 piece: When Barkskin fades, Savage Defense is automatically activated, and a 40 Rage Frenzied Regeneration is automatically cast.
4 piece: Activating Frenzied Regeneration and Savage Defense will cause a heal over time on yourself for 10% of your maximum health over 8 sec.
I'd say they both seem extremely weak. The 2p would in theory save you 100 Rage a minute, but what else are you going to spend it on? Mauling for maybe an extra 120k damage total? Healing yourself when you're probably topped up anyway, seeing how you just had a massive damage reduction? And the 4p - just what we needed, another Leader of the Pack..