When I write, I use a plan but during the course of the writing other ideas come to me. Some I dismiss, others I consider and a few will change what I am writing because I think they will benefit the story. I love this 'unexpected creativity' part of writing. It is like going for a walk. I leave place A and head for place B, but on the journey I see a pond which I stop to look at or flowers or a mysterious cave, however, I step over the dog poo. I keep heading in the same direction, but there are some pleasant distractions on the way. This is what has happened with Book 4.
The end is near, and it's going to be great!