Eu rail Geneva to Prague planning

You’d need 5 travel days for this; but if you’d like to go to Jungfraujoch with pass reduction, you’d need one more day. But there’s no six day pass. But you may book regular tickets for the cheapest trip and buy a 5 day pass (e.g. Salzburg to Wien is available for less than 20 € per person if booked early / right now).

Maybe good to know: Even the boat trips on both lakes at Interlaken are included with your railpass (travel day needed); it would even be possible to go part of your way to / from Interlaken by boat.

Another suggestion: For a much better travel experience, go from Genf to Interlaken via Zweisimmen and from Interlaken to Zürich via Luzern. 

Your pass is valid from Interlaken to Wengen on one side or Grindelwald on the other. You do need a ticket from there but you’d get 25% reduction (travel day needed). Be aware that it is a very expensive trip (expect about 550 $ depending on time / age).

It’s not mandatory, but with 4 persons I assume it would be nice to sit together at a table for 4 so it might be a good idea for Zürich to Salzburg, Salzburg to Wien (but there is a very frequent service with three to four trains per hour) and Wien to Praha. It’s not really necessary in Switzerland (especially if you should go 1st class).

Really no need to get one for this trip. Your railpass does give you flexibility and on most of your itinerary there is a frequent train service straight to the city center.