How many days in France?

FlixBus, Eurolines, OuiBus and National Express are the bus companies to check out. Keep in mind the trip will take about
7-9 hours, so taking a bus probably will shoot the better part of a day. Compare it to airfares at SKYSCANNER.com and consider the time and transit costs getting to and from the airports involved.
The optimal time planned for Paris depends a lot on what you're planning to do, but most find three days not enough.
If you plan to go to Versailles, are into museums or shopping, you'll need more days in Paris. Don't hesitate to plan for more days- especially if you are able to nail down good-value accommodations now for your dates of travel. And remember Bastille Day this year falls on Friday, July 14.

In general I agree with #1. You are going to have to do some research on Paris to see what is important to you. I would suggest you watch this video to give you an idea of just some of what is available. https://www.youtube.com/watch?v=n6ooG1ebQGg&t=63s I have spent over two and a half months in Paris over five trips and I would say more days is better. You might also get a Lonely Planet travel guide for Paris or go to your local library and look at others that are available. Be sure to look into the many day trips from Paris.

If you do look at that YouTube video, I suggest you look at others and also other travel videos of other places you plan to visit. They give you a good idea what is ahead of you and will help you make decisions on how to best use your time.

On our first trip we had seven days in Paris, and this included three day trips (Versailles/Chartres, Provins, and Château de Fontainebleau).

If you do not take any of these or other day-trips, then four days (five nights) in Paris is sufficient to get a great deal from the city, especially if fares are cheaper after four. Five days is not a huge advance on four days (if day-trips aren't on the agenda).

If budget is not that tight, I would take the EuroStar train from London to Paris - it's pretty good.

Of course four days is enough time in Paris.... but for what? I could have a great time in Paris in four days. But I think Paris is a real trap for many tourists. People who don't care about setting foot in a museum will spend a crazy amount of time just to get into the Louvre. Not because they like art, but simply because its famous. Same thing could be said for many monuments. Decide on the kinds of things you like to do, and then see if it's worth the time. For my part, I think going to Les Puces de St Ouen (a ridiculously extensive collection of about a dozen flea markets, block after block of outrageous goods), especially the Marché Vernaisson (the oldest and mostbeautiful market) is my favorite thing to do in Paris. I could care less about ascending the EIffel Tower which to me is especially ugly up close. But most people go directly to the Eiffel Tower.

Of course four days is enough time in Paris.... but for what?

We are basically talking about the length of a piece of string here.

But in an overall European holiday where time is reasonably limited in each sector, four days in Paris is "plenty" to see all its major attractions (and certainly five days is not especially better) - and I don't think climbing the Eiffel Tower of queuing for the Louvre are necessarily on the top tier either.

I'm not a big fan of markets in foreign cities - seems to me you've been to one market anywhere (even in your home town and ours is excellent), then you've essentially been to all of them. Maybe that's just me.

I am a bit confused

What precisely is confusing you? That four days "might not be enough time"? I'll repeat what's been said: enough time for what?

Basically you are creating a completely false dilemma for yourself, caused partly (it seems) by not knowing anything about where you are going. "Enough" is always a balance between the amount of time and the desired activities. Amount of time on its own is meaningless.

Someone might make a post on this forum and list 14 attractions they want to see and they have 2 days. OK, w can answer that. Two days will not be enough time, depending on the attractions. Either cut down the list or stay longer.

In your case, if you choose 4 days for economic reasons, then the simple solution is to make some priorities for what you want to see/do and then get to work and see/do things until your time is up. Then leave. It's as simple as that.

I've spent a half day in Paris, in transit. I have spent long weekends in Paris. I have spent a week in Paris doing little other than walking around. I have spent a week focused on the main art museums. It's all possible.

AS all above..depends on you,your interests and what you want to do there.....you need to do some research first.Then decide.

Everyone will have different opinions on the city and how much time is better (for me,I love the place and would spend as much time as possible,and I've already seen all the famous 'sights'...that doesn't necessarily mean you are the same).

There is a consistent theme (possibly dilemma) on this branch, it seems to me.

It is a lack of empathy or understanding of the position of travellers who cross oceans and invest large resources into their European holidays, which will likely be pretty infrequent - and possibly even just the once.

It's a totally different mindset required, compared to those who live in Europe and will have / have had many lifetime opportunities to visit Paris or Prague or Mykonos. For example:

I've spent a half day in Paris, in transit. I have spent long weekends in Paris. I have spent a week in Paris doing little other than walking around. I have spent a week focused on the main art museums. It's all possible.

For the OP, with limited time (and possibly one shot at it) asking whether four days is "enough" for Paris out of their overall trip length seems to me to be perfectly reasonable, and not that hard to answer within its own timeframe and constraints.