Keep notes from the start. Write down what the characters look like, and where things are. If you invent a town or something along those lines, make a map, even if it’s just chicken scratches on the back of an envelope.
By taking such steps, you will save yourself the effort of having to reconstruct these things by scrounging for details in the three novels, one novelette, and one novella you have already completed. And when the thing you’re trying to map is a faerie palace which (you have abundantly established) doesn’t correspond in a logical fashion to the city above it, you will be very grateful that you have saved yourself this tedious and problematic work.
If you fail to keep notes, you will use up all your scratch paper trying to find a way to make it all fit together, so you can then decide where and how to break it for the purposes of the fourth book. So be smart from the start.
In other words, don’t be like me.