we live in Zürich, Switzerland. The April temperatures are ussually around the 15 degrees celcius. The min temp. are by 6. so you might pack a good jacket. The swiss say "April,April macht, was er will" which means April, April does what he wants" . So its posible that you have one wonderful Sunny day and two days later its grey & raining. If you pack an umbrella/ rainjacket you may not regret it.
if you have any Other questions please just ask and we will be happy to help you.
BTW the excursions planned-Lucerne,Titlis- are really nice & a must see in Switzerland. We also stayed at the Hilton Doubletree during our visit in Amsterdam. They have exquisit chocolate chips cookies at the reception- we ask for it each day when we came back from our excursions :D