This is the epilogue to book 4. It's extra content with the love route characters of book 4 and the main girls of the love route in the older books.
Book 4 love route has multiple endings. After any of the endings you can continue with help from the snake to go visit Ember Island (book 4 love epilogue) to have a sex vacation with all the girls as I mentioned above.
This is the same physical location from Book 2 Love route (Ember Island). As I mentioned, after book 4 love route you end up back there with a bunch of women who all want to have sex with you. If you made the choices that CluckPover mentions in the post you quoted, and you got Ikki's scenes in book 4 love route, she will also be willing to have sex with you on the island (book 4 love epilogue).