When first starting a story it's natural to begin quite small and then slowly let it grow into something bigger. But, when planning stories (a practice which I am pleased to admit I don't practice), occasionally writers can overreach their own abilities.
“But I want to write a four-part quadrilogy about the life and times of my characters, their relationships and the adventures that they, their children and their grandchildren have over several hundred years!”
First of all, there's no such thing as a quadrilogy; it's a word that some movie executives came up with when they put
the quite awful Alien 4 on DVD.
Second, I can tell you right now that you're going to have a very difficult time writing a whole four books about one set of characters; granted, I can see the appeal of such an idea, but for someone writing their first or second story this kind of planning is almost completely doomed. There are several reasons why.
You might lose interest before you've finished even one of the books.
You're planning on them being actual books? As in, they're going to be published?
Is your world interesting enough to carry a dozen characters over hundreds of years?
Do you have a reason to have a story stretch over this many books? Are there overarching themes? Villains?
Lets take each bullet point and expand on it, starting from the top.
This happens to me often. It's never something I do intentionally. Sometimes I'll begin a story, write a few pages, set it down (in my hard drive), and forget about it for a year. Upon returning I realize that the story is either badly written, uninteresting or that it's just not worth finishing. Planning on writing four whole books seems like a long shot, even for me.
When you say books, I'm going to assume you mean books. Ignoring the fact that it's astonishingly difficult to get something published, printed and distributed, why does the story need to be told in a book? Until you've proven to other people (and yourself) that you're skilled enough to hold their interest for long periods of time, you're better off sticking with web publishing, or in my case, close-relatives-reading-your-stuff publishing. Take a writing class or two. Get some feedback before you throw yourself on the mercy of an editor. Get some thicker skin before trying to get anything published.
An interesting world? Overarching themes?
Carrying a story over three or four books is a tall order. Unless you're really meticulous and clever, your story will probably work itself out by the end of the first book. This means that it's not really a trilogy (one huge story with three big acts), it's more of a book series (a collection of stories that are related, but not necessarily supported by each other).
If these don't seem very different to you, let me use some real world examples of these kinds of books.
Star Wars (original movies) is a trilogy. When you watch all three, you're getting a much bigger story than any movie would individually give.
Terry Pratchett's Discworld series (books) are part of a series. You don't have to read the books in order, there are often completely different main characters from book to book, and there are so many books that you'd have trouble reading all of them in order. It's not made to be one story, it's a series of loosely-connected plots that take place in the same universe.
“But William,” you might say, “Tolkien wrote the Lord of the Rings trilogy all at once! If he can do it, I can do it too!”
Tolkien actually didn't write a trilogy; he wrote one colossal book. He had to split them up into six (three books with two sub-books in each) when he had them published because there was a shortage of paper after World War II. He didn't wake up one morning, thrust a finger in the air and exclaim “methinks I'll write a trilogy!”
Harry Potter is an anomaly because part of me wants to call it a series and the other a septrilogy. I guess you could read all seven books out of order, but then you'd be missing the complete story. Yes, each book has its own individual stories (which is part of the reason I love them) but the overarching plot (a certain Voldemort problem) really makes think of them as a septrilogy. I guess I answered my own question, and that is that the Harry Potters are more of a trilogy than a series.
So you have to decide for yourself if you want a trilogy or a series. I prefer to write in a series, myself, because it gives you room to make new (better) characters as your writing improves and you get a nice sense of closure between stories. If your universe isn't fleshed out very well, continuing books in a series give you ample opportunities to pile flesh onto your story's skeleton. Plus you can write something unrelated when you finish one without worrying about getting your characters out of a bind.
The first time you sit down at a piano, you're not going to play like Beethoven or Bach. You're probably going to plink around on the keys for a few minutes, get bored and leave. Writing is the same way; you need to practice and you need to be patient. You shouldn't expect amazing awesomeness to flow from you the minute you put a pencil to paper. Although you shouldn't expect awesomeness, you can at least attempt it. Just make sure it's something small enough to finish!