I bought a pass for zone 1-3 but need to go to zone 4 for 1 day. What should I purchase?

To travel beyond your Paris train pass (Paris Visite) coverage you can't extend its coverage for a particular trip.
One option is to ride to the maximum point of coverage of the Paris Visite 3-zone, exit the train at that station, exit out the fare gates, purchase two single use origin-destination tickets to your final destination in Versailles (one for the return leg) and re-enter through the fare gates using this single use ticket and complete your journey. 
 
The last train station covered by Paris Visite zone 3 for Paris - Versailles is:

• Viroflay Rive Droite along Transilien L departing Paris St. Lazare for Versailles Rive Droite train line
• Viroflay Rive Gauche along the RER C train line between Paris (many stations) and Versailles Chateau / Rive Gauche

 
On the return leg, use the other ticket you purchased at Viroflay Rive Droite to travel to this station from Versailles.  If you're stopped by a ticket controller and asked to show your fare, show the single-use train ticket, not your Paris Visite, until you're paste Viroflay Rive Droite.  After Viroflay Rive Droite, show your Paris Visite to any controllers that check. 
 
You don't need to get off the train at the station where you bought the single-use train tickets and re-enter through the fare gates using your Paris Visite.  The Paris Visite doesn't need to be validated on this particular trip since it's already active and within its zone of validity.
 
The other option is simply purchase two single-use tickets for the entire journey, start to finish, which allows you to skip exiting the train at the last station within zone 3, the limit of your Paris Visite.
 
See the Paris Train Fare Zone Map here.