According to a post on the PlayStation Blog from developer Monolith Productions' Community Manager Andy Salisbury, preordering the game for PS4 will get you the PlayStation Legion Edition missions. The exclusive missions will have you battling two more Warbands in Mordor: the Blood Hunters and the Flesh Burners.
The Blood Hunters, he said, are master assassins who travel in packs, and defeating them will earn you the Stinging Blade Rune. The Flesh Burners are a band of elite archers, and defeating them will earn you the Black Arrow Rune.
The missions included in the PlayStation Legion Edition are on top of The Dark Ranger preorder bonus you get no matter what platform you preorder the game for. It includes the Dark Ranger character skin and the Test of Power Challenge Mode. Both are different from the retailer-specific preorder bonuses.
For more on the game, check out GameSpot's latest preview.