Utnayan
So I broke into Act 4 this weekend. Like others have said, you have to beat 20 progressively harder Fort defense/assault missions.
Now I could either spend some time prepping for each fort mission by:
1. Exploring the game world and collecting captains by engaging through the handful of missions types.
2. Replacing weak captains by sending them against tough opponents in the fight pits that I capture when they destroy my pitiful captain.
3. Use whatever silver I have leftover from upgrading your forts to spend on loot crates, which are pretty good.
4. Training up the captains by playing Orke'mon, where I have to analyze an opponent captain's strength and weaknesses and pick out a suitable opponent from my cadre.
5. Also training up the captains by doing the different missions available.
#2, #3 and #4 are also pretty good ways to farm legendary captains, since you cycle through so many orcs very quickly. That list of tasks presents a nice break from the intense fort conquest fights where you're knee deep in orcs for a good 15 minutes. You don't need to spend hours doing it per zone per fight, just need to get a few captains, replace a couple captains and train a few captains before the next fort defense. Doing this will provide each area with a pretty beefy army as your opponent's strength grows.
You could also just lose the fort mission, and then do the pre-attack assassination missions to decimate an opponent's fighting force. Or you could just leather-ass it and defend a fort with whatever shit-stain orcs you have because if you're good and have a solid build with enough damage you can cut through enemy captains quickly enough to pull out the W.
But nah, I can't be assed to go through all that gameplay that is basically the entire focus of the game and is arguably more fun than the fort conquests themselves. I'd rather invest some $$$ on some Tokein-bucks to buy orcs to fill my ranks, and just slog through a single mission-type over and over until Sauron gives up trying to fight my credit card.