It's Okay To Take My Waifus To The Dungeon Waifu Dungeon

Cassandra Cain (Batgirl), Noriko Iwakari (Repeater), Julia Jacoby (Exalted versus World of Darkness), and Erza Scarlet (Fairy Tail) go to the Danmachi universe, plus a guy you don't care about but gets to dick them all.

(Content Note: There is no NTR in this story. One of the characters in the story has virtually no verbal filter and a wide variety of fetishes, so she mentions NTR at various points, but she will never have sex with a man other than the protagonist.)