The four piece bonus for druids at a glance seemed rather bad when I first looked at it. A 10% chance to timeslip a double duration regrowth or rejuvenation just seemed rather silly provided it wasn't much of a buff other than having to reapply it earlier. Then I got to thinking... would the 4pc bonus affect Swiftmend? Provided swiftmend works to heal a target for the maximum amount of a regrowth/rejuvenation, the doubled time of either would more than likely result in a doubled effect.
I really don't think that will be part of the bonus. I think the main and only purpose is to be able to have more Rejuvs out at any given time with less of a mana cost. Based on the tank four sets, it seems that there will be a lot of raid damage next tier. If there are any fights like Blood Queen in Dragonsoul, I think the bonus will work just fine though.
Swiftmend no longer bases its healing done on the Rejuv or Regrowth Swiftmended.
Since they rolled out with 4.0, Swiftmend has had its own completely independent healing coefficient. That also means Swiftmending either a Regrowth or Rejuv will heal for the same amount.