That's silly. Everything can easily happen in four more books.
Darktail is powerful, but he seems like a first half villain so far. He could easily be defeated in
Shattered Sky. Plus, all that stuff can happen at the same time. Dawn of the Clans had Clear Sky's reconciliation all through the second half (even if it was somewhat flubbed). And after OOTS, there's no way they're letting a story get dragged out across two arcs ever again. The whole OOTS thing might even have been the reason Vicky left the main books.
Also, not every SkyClan cat needs to come back. Some of them left of their own accord, and they don't need to return. A lot of it seemed like ways to write out the cats and slim down SkyClan without killing all of them off. Some of them could find their way back (like Kate said about one of the kits), but others can stay away if they want. We already have a ridiculous amount of lake characters.
Here's how I think it will play out.
- Shattered Sky: Alderheart finds SkyClan and brings them back to the lake. The cats battle Darktail and defeat him, but the bond between Twigpaw and Violetpaw is still strained.
- Darkest Night: SkyClan has returned to the lake, and Alderheart tries to help them move it, while the other Clans are conflicted over it. Alderheart joins SkyClan as their medicine cat. Also, there's a new villain, and even if they don't do anything here, by the end of the book, we know that they're the villain of the second half. It could be Onestar, but also maybe someone else. Violetpaw and Twigpaw still have a rocky relationship. They become warriors.
- Book 5: The cats face the danger of the new threat. Alderheart and Needletail make amends. SkyClan is feeling more at home at the lake. Somebody dies.
- Book 6: The time has come for Violet and Twig to embrace their destiny and determine the fate of the Clans. Everything depends on whether they can reconcile, or if they will face each other as enemies.
There's enough time for everything to wrap up.