DW and I will be traveling to Paris March 7-14. With a Saturday arrival, and the Navigo pass not starting until Monday the 9th, were looking at simply taking the train in from CDG by single tickets, and buying one carnet for sunday. (we'll use them all). For Monday to Saturday, I'm thinking get the navigo for zones 1-4 (which includes a trip to versailles) for all our travel. My question is the trip back to CDG on Saturday at 12. The navigo would get us to only zone 4, and CDG is zone 5. What is the most convenient method to purchase the ticket needed to make it to the airport on saturday. Is it simply a zone 4 to zone 5 extension or something like that?

There aren't supplement type tickets. You'd have to deboard at the last station in zone 4 and buy the ongoing ticket.
Your choice is to up the zones purchase to include zone 5, or just buy the RER ticket for that day. The Versailles r/t ticket is only around 6 euro, so it may not be worthwhile to buy even the 4 zones.

http://www.transport-idf.com/frontal...itresTransport

zone 2 C.O. + RER + carnet + Vers +RER = 50.8€
zone 4 C.O. + RER + carnet + RER = 55.7€
zone 5 C.O. + RER + carnet = 52.8€

I agree that if the sole reason you are paying for a zone 4 is for Versailles, I wouldn't do it. It costs more than the single ticket would, over buying just the two zone pass (it's ten euro more for zones 1-4 versus 1-2). Then just buy a regular ticket for the Versailles trip and to CDG.

I'd buy only a zone 1 and 2 Navigo and buy a Forfait Loisirs for Versailles, it includes your r/t fare plus entrance to the palais

Travelnut: do you have links to how you found the prices? I've been searching and can only get prices from blogs or other sites. I know I am searching the wrong sites, but I must be messing up understanding of the RATP site. Thanks so much for the information above, as well.

Yes, I gave the link in my 2nd post... You have to put the itineraries in, get the timetable, then click on the 'details' then the 'rates' buttons....