4 days in Switzerland Best "must see"

You will find many wonderful options in the Berner Oberland. First, think about what your plans are when you are there. Do you plan to go on the Jungfrau? With only 2 nights, I would stay on that side of the valley. Or are you Looking for scenic hikes and an interesting community with hotels, restaurants? Stay in Murren.

We stayed at Hotel Jungfrau in Murren and I can highly recommend it. Others have stayed at different locations in Murren and also had very positive experiences.

either lauterbrunnen or wengen for 2 nights. gets you away from Interlaken and up in the mountains a bit.

Traveling by train I would stay in Lauterbrunnen. For flexibility you want to be in the valley not on the edge. From Lauterbrunnen it is easy to to either the Schilthorn, the Jungfrau, Tremmelbach Falls or a number of other nearby sights. From Murren, to see anything other than the Schilthorn the first thing you have to do is spend precious time ( you are short on it) and money to get back down to the valley floor to go anywhere. I don't meant to sell Murren short, (there is a balcony railing on the valley side of the Hotel Alpina that probably still has my finger prints pressed into it) but it does have its logistic limitations. Have a great stay in the Berner Oberland,

Since you said "must see", my vote would be Murren. Scenery is absolutely gorgeous, the village itself is way cool (as is Gimmalwald), hiking the North Face Trail is not a real hike -- just a strenuous walk, and Hotel Jungfrau is a great place to stay.