Ice has good points. Split the early years and the later years, so that you get a sense of time shift that's as apparent in the real world as it is in the story. When you have a new story rather than, say, an arc or part, people automatically take that as expecting something new, something different, yet with a tinge of familiarity. I mean, take Xros Wars for example: you have Taiki's initial introduction to Digimon, then his temporary return to the human world, marking the end of part one of the series. The Death Generals arc then became the second part of Xros Wars, and now Young Hunters I consider a sequel, essentially, along the lines of how Adventure 02 was a sequel to the original. The one-year gap in the story practically seals the deal on this one. That you've already finished the four trials doesn't mean you can't split them from the rest of the book, provided you do provide a backstory before the trials, and you'll be able to write some semi-conclusion at their end.
"I have a reason for everything" without actually allowing readers to make sense of it themselves is annoying as shit. Why do you think people guess what happens on television dramas, mystery novels, or what will occur in the next book of a series? Because it's part of the fun! But they have to have enough information to do so. On a similar note, don't employ deux ex machina unless you have to, or the situation actually fits (unlikely if you're a good writer, unless it's a comedic story).
About the mention of writing in the middle and going from there, I totally advise against it. I only brought it up as a comparison, and I don't advise you to do the same. (Besides, what I meant by that was I would already know what kinds of scenes I wanted over the course of the story, and build it up to match that situation. It might sound kinda like a good plan, but it isn't really, because I tend to rush my writing and make what happens beforehand unbearable to read.)