I'm wondering what the advantages or disadvantages would be for using a 3 Act Structured plot versus a 4 Act structured plot. Could you please explain a little why a writer would choose one or the other. Thanks.
Well, I don't think a writer chooses-- I think the story chooses. Some stories fall nicely into the "action rising to the climax, then quick resolution" three-act schema, and some fall naturally into the climb to crisis, then long aftermath of the four-act structure.
So I'd say you choose the story, but then you don't have that much choice in the overall structure, because the story has an organic shape that you discover as much as you invent it.
So what's the difference? Well, novels tend to be 3-act (or five-act-- that is, the middle can break into three short acts, same basic structure), but there are plenty of 4-act novels. Films are more often 4-acts, I'm not sure why. It probably has to do with keeping the viewer interested in the middle, make sure they don't leave for popcorn without missing something. :)
Three Acts (I discuss this in depth in this article)
1. Set up, inciting event.
2. External conflict rises, rising action, many demands on main character. Ends on point of no return usually.
3. Crisis, dark moment, climax, resolution. The end.
Notice the "Crisis," the most dramatic moment in the plot, is at the start of the third act to precipitate a fairly expeditious ending. The middle act is usually the longest by far.
1. Set up, ending on inciting event.
2. Complications arising from the external conflict cause rising conflict, ending (near the middle of the story) in a crisis, where the worst that can happen happens.
3. Protagonist, who has lost all or been defeated in the crisis, starts a long climb back up. Could start again trying for the goal, or replace it with a new goal. (For example, in the first act, the goal could be protecting the leader, and then when the leader is killed in the crisis, the new goal could be getting revenge.)
4. Climax and resolution.
Notice that in a 4-act structure, the first act is usually set up (like the three act structure).
The second act is the build to the crisis. The crisis (the worst thing that can happen) comes at the very end of the second act, so almost directly in the middle.
The third act is the regrouping, the recovery from the crisis.
The fourth act is the climax-resolution, the working out of the conflict, or the triumph.
What's different is the placement of the crisis. 3-act: about 3/4 through, the very beginning of the end.
4-act: Directly in the middle.
Why? Well, there are some crises that can't be gotten over in a few scenes. Think of all the disaster films! They're as much about the recovery from the disaster as the causes of the disaster.
But it's not just disaster that can form the crisis. I had a book which ended up as four acts -- a romance, Charity Begins at Home. The first act, the hero and heroine met; the second, they got engaged, and then, suddenly, at the end of the second act, smack dab in the middle, the heroine broke off the betrothal-- the crisis. The third act was the hero trying to get a grip on this, and the fourth was his determined wooing her back.
Often the protagonists switch places at the crisis, as above, where the heroine was the main driver of the first part of the story, and the hero took over for the second part. I must say, I didn't CHOOSE the four-act structure, but recognizing it led me to emphasize the "breakup" by giving it an entire scene and making it more clearly a crisis for the hero (who thought they were both very happy :). That is, recognizing that the scene in the middle was the Crisis led me to make it BIG, and also not to get frantic because the end of the story had no crisis.
Or look at the Fugitive film. I'd say the protagonist of the first half (or at least the second act) was Tommy Lee Jones, the marshal, and the crisis was when he finally caught the fugitive, only to have him commit suicide (seemingly). The second half, Harrison Ford's quest to find his wife's killer because the driver of the plot.
So... I'd suggest that you go back to your story and think about whether that "worst moment" is going to happen towards the end of the story, with a slower buildup and quicker recovery, or in the middle of the story, so there can be more time to recover.
You tell me! All I know is, if you try to force a story that is naturally 4-acts into 3 acts, you're going to be frustrated.