Series: Rama

Why is it that so many sci-fi 4-book series are really 3-book series, but the 3rd book was so huge it had to be split into 2?  That's the case with Garden / Revealed. Actually, the story even continues from II, but that one is more self-contained.  Here is where all the action of the main story that I remember begins - the colonization of the spacecraft.  Many of the more memorable events happen in the next book, but this one sets up the scene for the final installment.