I am currently writing a science fiction/fantasy quadrology and would like to eventually expand on it. I have an entire world built around it and I would like to know the best way to introduce it. Prequels? Separate series? Guides? Through experience, which one best enthralls readers and/or best conveys the expanded world?
Tell more stories.
If you've built a world, put sentient beings in it and put conflicts in front of them. Let the world unfold in front of your characters, and let the characters talk about the other parts of the world, and the history of it, which haven't been discussed before. Write stories set long before or shortly after your existing series. Link it to your existing series via character, plot, place, or time (so that there's some reason for your readers' interest to carry over).
Mercedes Lackey has the planet Velgarth on which her Heralds of Valdemar stories take place; she has jumped around something like a thousand-year timeline telling various stories with various characters who may or may not interact (depending on how far apart they are). Many of her books are set within a period of three generations, but there are also quite a few set hundreds of years earlier.
Anne McCaffrey created the planet Pern. Most of her stories are set in a twenty-five year period long after Earth humans colonized the planet, but there's also one story about the colonization and few set before that group. The main links are locations and species (the dragons and their growing intelligence).
If your series get popular enough, you might be able to publish your Silmarillion or Codex, but to get popular, write more books in your universe.
Before trying to expend it into more than a quadrilogy, publish the first one.
Make sure the book is a success and the world appreciated before spending more time on it.
Even great authors sometimes create universes that do not resonate with the reader. Sometimes it is a game of try-and-miss.
Better fail with one book and spend time developing another universe or series, than spending more time than necessary in a bag end.
(Bag end, did you note the bad pun :) )