I'll be arriving in CDG March 28th Saturday and plan on being in Paris for 4 days including a day trip to Versailles.
Two options, one complicated but less expensive, another more expensive but easier.
Easy/costly solution would be 3 day 1-5 zone Paris Visite ticket for Sat/Sun/Mon, then a single-use Paris RER ticket back to CDG on day 4.
Complicated / cheaper option is to use single use RER tickets to go from CDG to Paris, Paris to Versailles (and back), and Paris to CDG. For Paris, single use Metro tickets as needed. Will cost about half the easier solution.