Hi Mimi,
If your arriving near the beginning of the week (Monday or Tuesday) and plan to stay within central Paris except for the one excursion out to Versailles, I would recommend buying a
Pass Navigo Découverte loaded with a week pass for Zones 1-2 and simply buy single "billet Ile-de-France" tickets for the Versailles trip.
The billet Ile-de-France is a single ticket priced for the zones you'll be traveling, available for purchase from any Metro ticket window. The current cost of
Paris to Versailles-Rive Gauche is €2,90, one way ("aller simple").
Purchasing a 4 zone Pass Navigo would be overkill for one trip out to Versailles, in my opinion.
The
Carte Paris Visite only comes in two flavours: 1-3 zones and 1-6 zones. The 3 zone Paris Visite card doesn't cover Versailles and the 1-6 zone is again overkill.
To help decide whether you should get a Carnet or a Pass Navigo, ask yourself if you would take two or more Metro/RER/Bus rides per day during your stay. If the answer is yes and you're arriving near the beginning of the week, it makes economic sense to buy the Pass Navigo, as you would be spending more on single tickets, even with the Carnet discount.
Let me know if you have any more questions. Otherwise, have a great trip. Sept. is probably the ideal time to visit Paris.
Ben