Wow, I appreciate all of the responses! There's a lot of good stuff to think about here 
I'm hopping back into the writing tonight, but I'm leaning toward @revisionstudios, @punkarsenic, and @Rhombik's suggestion of adapting some of the traits of Character 4 into Character 3, (or potentially the opposite, dissolve 3 down into 4; I'll have to re-examine the scenarios and see which seems most applicable).
@storytimebiondi - This outlook was particularly useful, I think! Due to the scope of the project, I've aimed to spin the story around a single character's arc, the main character. A few of the other ones are key in MC's development, but I wound up with more characters than I actually needed to make the party "look right" (in terms of # of people, variety - according to an arbitrary picture of those things that I envisioned haha).
Further, what you said about developing the character more, to find where they can make a decision that alters the path of the story is really interesting! In an earlier draft, there was one scene where the character was actually a key player and had a large impact. However, I ended up editing the scene down quite a bit, as the way it was originally written would take up way too many pages relative to its importance.
I think this issue that I've encountered is actually an interesting lesson in short vs. long story telling: If this were an arc in a longer story instead of a short one-shot, I wouldn't necessarily have questioned using the longer version of that scene, perhaps developing it into its own chapter, because it would have been a key moment for the character in question to develop. However, since I really want to focus on the strong development of a single character, having that longer scene isn't necessary, and thus the would-be important 4th character became irrelevant. I'm sure I could tackle the story at a super intricate level to force some relevance into that character, but outside of being an interesting exercise I think that would start to pull away from the story I'm trying to tell.
Rambled a bit there, but this was a lot of useful feedback
Even the posters I didn't specifically tag haha. Definitely open to hearing more perspectives though!