CDG to Marais

The RER for 4 of you, then a taxi from Gare du Nord to the Marais, will cost about the same as a taxi from CDG. Barring traffic problems, of course.

Edit: but as I just realized, it will be Friday afternoon and the traffic will certainly cause problems. Better to take the RER.

Unless you are all travelling with nothing more than carry-on, I suggest you take a taxi from CDG. Why make a transfer when for just about the same cost you can have door to door service. A taxi from CDG to the Marais is about 50 euros. There is a 3€ charge for the fourth person and a 1€ charge for each bag placed in the trunk after the first one. Go right to the taxi rank outside the terminal. Don't accept a ride from anyone who approaches you inside the airport.

Depending on which end of the Marais I was staying at, as a single traveler, I have chosen either the RER direct to Chatelet Les Halles (for a hotel near the Hotel de Ville, €10 per person) or the "Les Cars" Air France bus #4 direct to Gare de Lyon (but at €17.50 per person, that won't save your group any money). Rick cautions against trying to reserve a car service to match your airport arrival; a regular taxi is much simpler.

Here are three choices:
If you want to do as Laura says, take the RER to Chatelet Les Halles, it's a fairly short walk to the Marais if your luggage is light. Head toward the Pompidou center when you come out of the RER and keep going. If you choose to get out of the RER at Les Halles, it's an escalator ride to the top.
If you choose to transfer to the line 1 Metro at Chatelet and get out at St. Paul in the heart of the Marais, it's a short connection with short little escalators between the RER and Metro. Line 1 and RER B are on the same side of the Chatelet Les Halles station. St. Paul has an escalator to the street. The RER will get very crowded between Gare du Nord and Les Halles. Line 1 will be crowded.
A taxi is easy but you will run into rush hour traffic, even outside Paris, and the meter will keep ticking away. With four people, I'd normally jump in a taxi too, but my only hesitation is your arrival time pushing you into Friday rush hour.

There is absolutely no need to take 2 taxis from CDG ever. The dispatcher on the official queue will match your party with a cab that will hold you. There are cabs that take 7 people and luggage. A regular cab can take 4 and luggage but it is tight.

With 4 people it makes zero sense to pay 40 Euro to Nord and then take a taxi for another 15 or so when you can take one for about 55 or 60 from CDG straight to your hotel.

Cabs that take 7 and luggage? Wouldn't that be called a bus?

"..... 7 and luggage ". Where did that come from? There are only 4 people.

I'm pretty cheap, and I hate sitting in traffic, and I would still opt against the RER in this instance.
But don't dismiss a pre-arranged cab rather than just walking up to the taxi ranks.
For instance:
https://www.lecab.fr/nos-services/taxi-aeroports-ile-de-france.html

While it's rush hour, it's likely that the flow of traffic will be out of the city, not into it, so taking a taxi or Les Car Air France bus could both be good.

If you only have carry on luggage, RER is pretty easy. If you can connect to the metro at Gare du Nord, that's better than Chatelet-Les-Halles.

The bus is very comfortable and stops at Gare de Lyon. You can probably get a bus that takes you close to where you're staying in the Marais. That's a lot easier that RER/metro if you have more luggage (no stairs, very little walking). It's about twice as much at €17 one way.

A taxi will cost more. How much more depends on city traffic.

We used InterShuttle Prestige when we arrived at CDG to our rental in the Marais in late April. Then again to go to/from CDG-Sevilla-CDG in mid May. We would use them again.

The link I posted above locks in a fixed price regardless of traffic, and they allow 3 'valise normal' in the trunk. If your group of 4 isn't oversized and doing carry-ons only, I think you'd be fine.
https://www.lecab.fr/nos-services/taxi-aeroports/aller-aeroport-charles-de-gaulle.html

And it can be boring to criticize others before reading other posts completely:

There are cabs that take 7 people and luggage. A regular cab can take 4 and luggage but it is tight

Kind of high jacking a bit.. But if you were getting off at les Halles as final stop (still with 4 people and carryons) - would you opt for RER or taxi?

Kim - depends on the amount of luggage and the time of day. I believe there's at least one exit at Les Halles with an escalator to street level, but it may be hard to find and require walking for 2-3 blocks in the metro itself. Maybe also how far you have to walk from Les Halles to your hotel with your luggage.

Janet's point was that there are taxis that take up to 7 people --- i.e. given that taxis can take even more people than you need for your party, there are definitely taxis that can take 4 people as per your requirement!

To re-emphasize what she said, the taxi rank person will find the right-sized taxi for you. As they come up, some are smaller and some are bigger. It works pretty easily. There is no need to reserve ahead.

And for the price differential per person between the RER and taxi, as well as the comfort differential, there's no way I would schlep through the RER if I were arriving with 3 other people. Taxi all the way.

Has anyone used Le Cab? Seems like a good option but would like to hear from anyone who can speak first hand about the service before booking.

Ben, there are 2 posts on this thread from avirosemail recommending Le Cab.

If I'm by myself at CDG, which is usually the case, the right option is generally the Cars Air France,
but if I'm in a small group, the set-price cab is usually the way to go -- not as pricey as a car service with someone standing there with your name on a sign, and still a pretty good deal compared to 4 RER tickets. I've been burned by metered taxis in heavy traffic or at after-hours rates, so pre-paid is a welcome choice.

I can think of several airports where the top few taxi service providers were noticeably pricier than those in the next tier below, so as I have mentioned: I am cheap, so if I can get a bargain, I will. Use Le Cab as a yardstick for further comparison, maybe?

The RER isn't as appealing to me because I find the public address system hard to understand, the carriages not very cushy, and the riders a little irritable -- I'll use it for a clear, simple segment when I'm fully awake, but not while tired or drunk.

Well CDG isn't a place where there are tiers between taxi companies (using meters). All companies with taxis using meters follow the French law and regulations (and those in place for Paris and the airports) as to fixed fees for pickup, additional bags, additional passengers, and rate per km depending on the time of day/week category A/B/C.

This poster this morning had a different experience with a Paris taxi:
https://community.ricksteves.com/travel-forum/transportation/cabs-at-paris-train-stations

Indeed. But that's a very unusual situation in Paris--a rogue. In forty years taking Paris cabs, I've never encountered such a thing. A visitor wouldn't know to tell the driver to turn on the meter right away, unfortunately. BTW, the cab number and phone number where to report suspicious transactions are on the left rear window.

We used Inter Service Prestige recently to get to and from CDG. 50 euros and well worth it. Stress free travel, they even handle your luggage for you if you want. Great company!

We used a combination of Air France Bus and taxi CDG to our Marais hotel. We used the hotel arranged shared shuttle back to CDG.

China is suggesting that their citizens NOT take the RER because of safety concerns:
http://www.dailymail.co.uk/travel/travel_news/article-3005370/Chinese-tourists-warned-stay-trains-Paris-Charles-Gaulle-Airport.html
Chinese embassy in Paris took the rare step of warning holidaymakers
It said four Chinese visitors have been robbed since mid-February
All four incidents occurred on the RER train between Paris and the airport
Embassy is telling visitors to use buses, taxies or 'other transport'

It's off the subject, but to add to Edgar's post:
It's well known that Asian tourists, particularly Chinese, are the preferred targets of pickpockets because they are known/believed to carry a lot of cash. North Americans, we're not the favorite. In fact, there are Chinese police working in Paris this summer, along side the French police. FYI, Romanian police have been in Paris for several years now too.