The best and quickest way to gain lots of XP in Act 4 is by losing your keeps instead of defending them. You earn a lot more experience by retaking a fort than you will successfully defending it. You get approximately 50k XP for successfully defending a fort, while you get approximately 160k XP for failing to defend a fort and then retaking it. For more XP, add White Gems to your armor for a small XP buff. Upgrade them for even more XP. When losing a fort, you do not lose Warchiefs or Overlords. Overlords are captured, so if you want to keep them, you will have to complete a rescue mission later. You can send out Orcs to save him. To make things easier, replace your good Overlords with useless ones before losing. There is (currently) a bug that makes the rescue mission sometimes never appear. To avoid potentially losing your best Uruks, leave some spaces open and swap terrible Overlords into your fort. For every fort defense quest, simply lose the fort and retake it for around three times the regular amount of XP you would earn.