Feb 1, 2016 - 3:12 PM
Lauterbrunnen is beautiful, you'll definitely enjoy your stay there. Please study the "What to do" sections of our Jungfrau region and Bernese Oberland pages for recommended activities. Note that May isn't high season, so not all trains and cable cars will be operational yet. The main ones will be though, and there's more than enough to do for a 4 days stay.
Which pass is best can only be told for sure once you have a finalized list of plans. For now, I suggest you look into the 4 day Swiss Travel Pass. It will cover all regular train, bus and boat journeys, including the long legs Geneva-Lauterbrunnen and back, and discounts on mountain trips. By the way, I recommend to do either your inbound or outbound trip (or both) by Golden Pass via Montreux and Zweisimmen, rather than the quicker route via Bern. The Swiss Travel Pass will cover the Golden Pass route too.
An alternative is the Swiss Transfer Ticket Combi, providing a free return trip Geneva-Lauterbrunnen-Geneva plus a 50% discount made on trips during your stay in Lauterbrunnen. However, the Swiss Transfer Ticket requires you to use the quickest/cheapest train route, and that doesn't include the Golden Pass.