I know you're not a native English speaker, but in many circles of general nerd culture in America and on the Internet, there is a phrase for what you're doing right now: your spaghetti is falling out of your pockets.
In all seriousness, though, you seem very anxious about your situation and you should take a step back and relax. You say you're good at improv, but it looks like you're already sweating bullets because your party got sidetracked instead of getting to the quest you wanted to take them on.
I'm going to put my direct answer to your question first and some general advice afterwards.
Your players probably won't miss their characters. Why? Well, you generated them, and even wrote up their backstories. These characters were never made by the players playing them, and while they might have grown attached over a few moments, I find it extremely unlikely that they'll shed tears over a party that lasted for four sessions... that they didn't even make themselves. Even if the characters are extremely cool and interesting, that element of personal attachment just isn't there, which allows the players to truly say that they helped create an exciting story with interesting characters. This obviously varies from person to person, but again, I find it very unlikely that your players would have grown very attached in this case.
Go out with a bang. You know that amazing epic encounter you were saving for the climax of the plot arc? Yeah, run it now. You'll have to make some tweaks because you obviously aren't quite there yet, but get them there as fast as possible. Feel free to kill off PCs or even have a TPK at this point; memorable deaths are often much better than "and then they lived happily ever after."
Here's a few things to keep in mind when starting your next campaign:
Your content will come to light eventually, and it will be good. The quests you've designed will always find a way to come forward. Even if your plot arc is entirely ruined by something the players did, you will be able to recycle the content you made but never ended up playing, and I encourage you to do so for your new campaign. The only things that are truly lost are "hard" materials, like NPC stat sheets, etc.
You're in control, and therefore, you set the tone. Sometimes, it is best to take a page out of Gygax's book; after all, this is your campaign, and you put a lot of effort into it. Obviously you shouldn't take the entire preface from the AD&D DM's Guide to heart, but there is a point where a DM should draw a line in order for there to be some kind of structure, assuming you want your campaign to go anywhere. If your players are goofing around and killing NPCs for no reason, or making light of important people in-character, then they should be ICly punished for it; reprimanded for insulting a nobleman, pursued for attacking innocents, etc. It is also very possible to play a serious game in character and laugh until you're blue in the face out of character. This frequently occurs in the Dark Heresy games that I've played and ran.
You had better get used to murdering your darlings. This is a phrase commonly used amongst writers and creative designers everywhere in the U.S. The phrase means that you'll have to scrap ideas frequently, including ones that you really, really liked, so you had better get used to it. The saying is intended for use in the writing, film, video game, and other industries where a publisher or producer oversees your work, constantly telling you what can and can't make it to the final product based on time and expenses. However, it works just as well for when your ideas can't make it to the game because your players did something insane. And, on a related note...
Plan less. I don't know how much effort you're putting into writing everything ahead of time now, but you might want to ease up on that. From what you're telling me about your role-playing experience, it seems like you've been playing in a "safe" and slow environment where you rarely, if ever, have to scrap or re-do material. This happens literally all the time in regular tabletop RPGs, thanks to the insanely unpredictable nature of 4-5 different people working together. It will save you a lot of anguish if you lay out a basic outline of what's going to happen and then add the details once you're sure the players will be arriving there next session, or maybe two sessions later.
It seems like you've learned a lot already OP, which is great, but scrapping a campaign after 4 sessions (and while your players are all enjoying it) is something you should really avoid. If everyone else is having fun, consider either shaking things up a bit and changing your own notes, or coming up with a way to set them back on track, which doesn't always need to feel contrived or railroad-y.
EDIT: Well, now that SevenSidedDie has made that edit to your post, there are a couple of details that I didn't quite catch before, no offense. Since you said you have a month between each session, it seems like you might be over-planning because you have a lot of time between sessions. Heck, you might even consider having more frequent sessions, if you can't stop yourself from overthinking it in the intervening months. If in-person is not an option, use Skype and/or Roll20.
When a player is hogging the limelight like this, the way to deal with the situation is to stop encouraging them. The player is getting their fun by having everyone's attention focused on them (see this question for a similar situation). (This isn't a bad thing, by the way! It just means you have to make sure that the rest of the group gets their fun, too.)
Don't Reward Indecisiveness
Right now, it sounds like the entire group is (unintentionally) rewarding the player by watching when he's roleplaying, letting him stall, and otherwise paying attention to him. So the first thing you need to do is to stop paying attention to him. That sounds harsh, but doesn't have to be.
If the player refuses to make a decision in-character, the DM should simply say, "Okay. While you're thinking about that, I'm going to switch scenes to the other characters for a bit. Let me know when you've made a decision." Then the DM swaps focus to the rest of the group and ignores the player (except to answer the occasional reasonable question, such as a request to make a knowledge roll to help make a decision). This is going to be difficult for the DM, because the player will immediately do everything they can to get the attention back - asking lots of questions and demanding immediate answers, trying to RP more stalling, etc - but it's up to the DM to repeat, "I've given you all the information your character has. Let me know when you've made a decision." Then the DM must return to ignoring the player until they take action.
Similarly, the other players can help by not letting the player sidetrack or delay discussions. I've DMed and played in a number of games where one person (in- or out-of-character) couldn't make up their minds about what to do. If the rest of the group couldn't convince them to take an action in a couple of minutes of discussion, the other players would just forge on ahead, telling the character IC, "Catch up when you're ready, or stay here and guard the horses, it's up to you." Then either the player would say, "Screw it," and go along with the group, or the player came up with a reason to stay behind and sat out the following scene(s).
In cases of slow, drawn-out RP, it's perfectly reasonable for the NPCs with whom the character is speaking, to get impatient and interrupt the PC, talk over him, make assumptions, or otherwise do any of the things actual humans do when they get impatient. So if the player is giving a drawn-out William Shatner speech about "My... disease... (cough) is getting... (cough) a lot... (cough) worse... (cough) and I need...", most NPCs are likely to interrupt at this point with, "Yeah, yeah, you need the local cleric. Two streets over, third building on your right. Have a nice day!" Then the NPC will back away quickly and refuse to speak further (because who wants to spend any time around someone who's obviously sick and coughing up gross phlegm?). Note that the NPCs don't have to - and shouldn't necessarily - answer the question that the PC was actually going to ask. They're impatient and grossed out; they're going to make an assumption and skedaddle before the PC can correct them. The player will quickly figure out that if he doesn't get to the point, NPCs won't stick around long enough for him to talk to.
Do Reward Fast Movement
Rewarding the player when he acts quickly, whether by making a decision, roleplaying smartly, or otherwise playing in a more group-friendly way, will help encourage him to do those things more. For example, have NPCs be more helpful if he gets to the point quickly, or compliment him on his swift decision-making if he doesn't dawdle.
It's going to take a while - and a lot of work on the entire group's part - to encourage the player not to hold up the game for everyone. The player may respond to initial attempts to speed things along by being louder, or going even slower than usual. Ignore him, don't give him any attention, and don't let him slow down your game, and he'll eventually figure out that being a valued party member is much more rewarding than being the guy everyone is ignoring.
Best Answer
I want to preface my answer by saying that I understand where your impulse is coming from, that I respect it at least in part, and that I share that impulse. But with that preface, I must in good conscience push back against the frame of the question:
Consider Reining In Your Urge To Discourage
I respect and share your impulse, here. I really do.
Every time a player of mine goes off and does something like this, I have a little (or not so little) frisson of fear: What if they're going to break something I had planned? What if they're power gaming for an advantage? What if they touch my GM stuff?!
After a long career in GMing, though, it's rarely been an issue in the way I fear. Are power gamers gonna power game? Yes. But it turns out they're fairly easy to detect and shut down. Will the players occasionally do something that wrecks a plan? I can't actually think of a time that's happened, but I can think of several times that a player was pushing in one genre direction and I was pushing in another... which can be an issue.
But on the other hand, having engaged players who want to contribute to the setting is a blessing in many ways. First, it just means that they are active and are engaged which is a great thing in and of itself. Second, it means that in some sense, they're taking a little of the creativity burden off your shoulders. Third, when players do this in good faith, they are telling you what they are interested in! They're directly telling you something other GMs have to read tea leaves and pull teeth to get from their players. And fourth, sometimes (not infrequently!) a creative and engaged player will come up with something really good that you wouldn't have thought of that you can turn to your advantage.
Still, there are at least potential issues.
Meet Them In The Middle, With Approval
What I do, therefore, is to make clear the following things:
I've found this works pretty well. I'm only GM of my experience who explicitly spells out the fourth point, but that's certainly how it's worked out when I'm a player in a game that encourages player contributions. It definitely works from my side of the table when I am GMing.
Addressing some issues that have come out in comments, which may or may not make it into an edit of the question: I must stress that all of this regards background information, not questions of current NPC ownership or behavior. Different games address this differently, and even within 5e different GMs will have different levels of tolerance. At the very least, influence on current NPC actions strikes me as a qualitatively different question. (And if you think the player is angling to backdoor that sort of thing in through the submitted background, well... that's what veto power is for.)