Is Unbreaking III not enough for you? Then this reward is just what you need!
What you get: A reward book which can be consumed (self-service) to add IV Unbreaking levels to any Diamond Tool (Pickaxe/Axe/Shovel/Hoe), Shears, weapon (Diamond Sword/Bow/Crossbow), or Elytra. Yes this will stack with existing Unbreaking levels, you may go as high as you dare!
This reward cannot be applied to armor.
Why +4 UB levels?
Because a UB VII(7) item will last twice as long as a UB III(3) item and that feels nice! With +4 UB levels every time you apply this reward to an item it effectively gives you the added lifetime of another UB3 item.
All reward books will be delivered to your in-game reward archive (/rewardarchive) and can be put back in (/rewardarchive put). All reward books in your reward archive at the time of a map reset will carry over to the new map. Once a reward book has been used in game to redeem the reward that book is consumed.