Personally I would take a taxi into town if I was with a party of 4.
I disagree. Particularly with a large party, a van has some major advantages:
More comfortable, roomy, no squeezing.
Plenty of room for luggage.
Door 8 more convenient from baggage claim area than Door 12.
No waiting in lines, van is reserved for you.
No worries about a meter running, price is flat rate. No extra charge for luggage.
For me, it's a no brainer. The service I've linked to above has given us terrific service on our last several arrivals into CDG.
Re: Your itinerary--
DO NOT DO ALL OF THESE CITIES. Your memories of the trip will be a blur, and you will spend more time commuting between the cities than enjoying your vacation! At the most, do Paris and one other city. You can do the rest on your next trip-- don't worry, you will be hooked on European vacations forever. You will be back.