Berner Oberland Regional Pass 4 days or 6 days?

I do not have access to my bookmarks right now, but there are a coupel of differnet passes. Some cover some lifts and trains, and others have different coverage, so look at that. I think We had a Jungfrau Regional Pass last year and it was good, but it did not cover the cableway from Murrem to Gimmelwald to the valley floor.

The passes are expensive so you hate to buy more than you need. They do cover the train from Interlaken to Lauterbrunnen, so it that is how you are arriving, a pass for your first day is useful all the way from Interlaken to Murren via Grutschalp. You will also need a pass, most likely, the next three days, so a 4 day pass is pretty likely. It will depend on how you are getting to Munich as to whether a 6 day pass is overkill. How many segments covered by the pass will you take on the morning of the 21st? What do they cost full price without a pass? Will a pass discount enough to make buying the 6 day one worthwhile?

I suggest you plot out day-by-day the segments you thin kyou might take, and especially the arrival and departure routes. In Interlaken when you arrive you can consult with the TI about the best pass options. If I remember I will look up the links at home tonight and send you some sources that might help, too.

I don't like the idea of purchasing a pass that limits me to an area. You could arrive into Lauterbrunnen have the most miserable weather for 3 days (I hope not). But in that case you might like to travel to some other destinations in Switzerland and with the BO pass you'll have to buy additional tickets to get somewhere else. I don't like the idea, but maybe it's ok with you.

Are you arriving into Switzerland and Lauterbrunnen via train or car? If you're training then I would probably sugges the Swiss Transfer Ticket + Half Fare Card Combination Ticket:
https://www.sbb.ch/en/leisure-holidays/holidays--short-breaks-in-switzerland/swisstravelsystem/swiss-transfer.html

It will cover your Paris->Mürren trip from the Swiss Border forward. It will also cover you from Mürren to the Swiss border when you depart for Munich. While your in Murren (or anywhere in Switzerland) ALL of your various tickets will be 1/2 price.

I prefer Tim's suggestion of the Swiss Transfer Ticket + Half Fare Card, which you can buy either for home delivery or in Basel train station.

If you do choose the Berner Oberland Pass, then the price difference between 4 consecutive days and 6 is 60 Swiss Francs per person. From Muerren to the end of the pass coverage at Luzern costs 44 Swiss Francs, so if that is all the travel you need to cover on the fifth day, the separate ticket will be cheaper.

From Paris to Basel and Zurich to Munich, you can get significant ticket discounts by reserving about 3 months before your date of travel through the French and German web sites.