How flexible are you on the time horizon ? How much in percentage of your house do you already have today and ho much is what you plan to earn during those 4 years ?
The best answer is maybe bank’s bond at the Caisse d’Epargne d’Aubonne at 1% for 5 years for example https://ceanet.ch/conditions/ . If you choose a bank account with 0.00000X% instead, then maybe you can invest 20% in ETFs now and should the market go down, wait 12-18 more months until you have fresh money to use instead or maybe the market goes up again during that extra time.
Edit : You can also put those 20% in swiss residential real estate exchange-traded mutual funds (or 10% ETFs and 10% real estate funds). They are safer than stocks, they are somewhat corellated to the real estate market, which is what you want to buy with the money. And the probability that both your ETFs and your real estates funds goes down at the time you want to buy is lower.