<<"on the fourth day depart from Interlaken to Zurich airport ( also included with the pass, How long does it takes?) to be at the airport around 7pm.">>
Perhaps you missed my reference to the timetable in an earlier post? There are an impressive number of frequent trains to your destinations. It can help you be flexible with your itinerary, especially since you have decided to get a Swiss Travel Pass. And, it answers your question about the return to Zürich Airport.
it is far enough in advance that you can still be flexible about your hotel arrangements.April is not a busy time for the hotels.
If I were doing it, I'd definitely go straight to Luzern upon arrival, and sleep there the first and second nights. It is an easy and short trip. About 40 minutes longer than the trip to Zürich.
Here are some data, assuming arrival at Zürich airport in the afternoon of Monday April 4, 2016. Luzern is only a short train ride from the Zürich airport, with service every 30 minutes. Every other train has no change at Zürich main station (see timetable) , and goes directly to Luzern. I know that some of them, and I think that maybe all of them, finish their trip at Luzern.
I have attached 3 screen grabs from the timetable, to show an option for your itinerary consistent with your goals and time schedules. It would permit a full day in Luzern to visit both the mountains you noted (personally, I would only visit one) , the scenic train ride on the Golden Pass Route in the train called the Luzern-Interlaken Express with panoramic windows, and 24 hours based in Lauterbrunnen, not Interlaken. Annika mentioned that possibility. It is possible that you could even do some sightseeing in Luzern when you arrive; if not, you could visit only one mountain and spend the other half day in Luzern itself. Lots to see.
I have attached two other screen grabs which illustrate the frequent trips available to go from Lauterbrunnen to either Kleine Scheidegg by extremely scenic cog-rail train or to Schilthorn (PIz Gloria of James Bond fame) by multiple cable cars, and return within a 3 hour time frame for either one. Those two screen grabs overlap PM 6th with AM 7th to show you when service stops in the afternoon of the 6th , and when it begins AM 7th. Obviously, it continues all day. I did not show return trips, but you can figure that out.
That way, you get one mountain in Luzern and whole bunch of mountains in the Jungfrau region.
Enjoying any high mountain is dependent on clear weather. Clear weather is not guaranteed in April. ;-(
For that reason, many people schedule more than one day near the Jungfrau; the weather changes quickly in the mountains and a two day stay gives a better chance of good weather.
Luzern has plenty to do even in cloudy weather. I would do Luzern first and while there, carefully - very carefully - watch the weather reports and webcams for the Jungfrau region. Your hotel in Luzern could help with that; so could the excellent tourist info office by track #1 in Luzern station. If the clouds are low and thick, you could take a boat ride on Lake Brienz or Lake Thun, visit Thun, visit Bern, or look around the Lauterbrunnen Valley. Or, not even go to Interlaken/Jungfrau region, although that could depend on your hotel arrangements. In April, the hotels will not be full, but some will be closed. Early April may be better than late April....but, you might find out if a hotel has plenty of rooms and not book ahead....not my preferred way to do it, but frequently done. Check with the tourist office in Interlaken by phone.....they would know about hotels in or near Lauterbrunnen.
For what it is worth, that is what I would do. It probably differs from your plan because - I'm guessing - I and the forum moderators have a better understanding of the travel times and travel availability than you do. Review of your options with the timetable can help you understand my thinking. Please understand that I am only making a suggestion. Obviously, it is your journey, not mine. ;-)
No matter how you organize it, you'll surely enjoy it. There is a a lot to see and do, the country is beautiful, and the transportation system is well organized to help you.